I feel like one of the main things to keeping critique useful is keeping the criticism at the same level as the artist’s skill, a professional painter of 10 years and a 15 year old who just downloaded ibisPaint are going to need different kinds of advice and feedback; telling a beginner things that a more intermediate artist would have trouble understanding is confusing and telling someone who’s been drawing for years and years the basics can be patronizing.

I've always been big on phrasing regardless of context especially having grown up on the phrase "it's not what you say but how you say it"

I personally feel that some of the best critiques are able to point out things done well, acknowledge flaws and then give guidance or direction on how to improve those errors. Not everyone has learned how to do this thus you're prone to get a lil bit of all the types of feedback thepenmonster mentioned. It's why I think it's also kind of up to the individual to know how to parse through what feedback they've received and know how to weigh their value.

Not all criticism will be constructive and not every individual knows how to phrase things. Not to mention that only so many folks will come into a critique with full context on how you do things or what you do or don't know which can be frustrating at times but you learn how to go with the flow (thus the bit about parsing feedback you get)

Critiquing, as least in the forums and discord, isn't very structured, with is understandable given the space. Some people give really in depth critique others prefer a brief overview on what can be improved. It really depends on the person.

I don't think in seeking out criticism you should only seek advice from those who would conceivable "know better" than you, though I understand why that would be preferential. Seek advice and criticism from all types of people from all types of backgrounds and skill levels, if only to expand the amount of insight you can receive for your work. You may find that a layman may notice something that you otherwise would miss.

But as for best practices as a critic, I don't think there's particularly a right or wrong way to "critique", you just have to be considerate of the goals in mind when giving the critique: both yours, as the critic, and for the receiver of the critique. Like @AmeTsunami said should be considerate of skill level and expertise, both yours and the receiver. You should be goal oriented and openminded. You should try to consider why the person you're critiquing made the decisions that they made. Be as detailed as you can, give examples of faults and give suggestions for improvement. Even if you assume to know more that the receiver, do not talk down to them talk to them. Be encouraging, be ready to answer questions, and most importantly, be ready to not get listened to. Despite the idea that critique is a teaching tool, it's still on the basis of personal opinion and biased insight. And because the critic and the receiver are not the same person, there is a chance that the receiver will, ironically, not be willing to receive the criticism given to them, not matter how poignant it may be. And these are merely suggestions as opposed to a strict guideline on how critiques should be carried out.

(I'm not giving these suggestions in reference to the space of professional criticism, like that of an established movie critic commenting on a seasoned director, but more in the informal space of creative mutuals.)

Despite thinking there is not 'best way to critique', I am of the belief the best way of thinking going into giving a critique is getting ready to be rejected. Your suggestions, however valid, are not rule or law; they are not infallible, at least to the receiver. They can rebuff your opinions if they so chose since their work is theirs and they are free to lead it into whatever direction they choose, no matter how unfortunate that can be sometimes.

In most college classes you will get the critique sandwich. Start with what is good, middle part you put what isn't working, and finish with a strong positive. People seems to take it better that way.

The person who is looking for critique most be open for critique and has to roughly know
what they are looking for. That´s 2 points where online critique fails.
The other thing is that many people who give online critique don´t have a clue or don´t
know how to give critique in a way that helps the person asking for critique.

I had helpful, free online critique when I asked detailed questions so the people
know what to focus the critique on but I also had to filter the comments. You don´t
know what you are getting when you ask a bunch of strangers, you can theoretically
get answers from people who are drawing since one week and think they are experts
and tell you "never use reference! pros don´t use reference and draw everything
from their artistic imagination"

The best way to get critique is to have a (paid) mentor. Someone you chose and
who does something you want to get good in

I know I say this a lot, but I think feedback is best treated like UX (User Experience) research.

Think of it like this: Most people who use your product, ie. read your comic, are users. Users are very good at finding things that don't work well, whether you're testing a game, navigating a supermarket or reading a comic, because there are a lot of them and they go in just wanting to use the product. If they keep getting stuck on one boss because the attack pattern isn't obvious, if they can't find which aisle has the ketchup, if they keep getting two characters mixed up, they're not incorrect for saying so. Users are great at finding problems, so if they say "I couldn't get into it, it was a bit slow" or "IDK, I just don't like this comic's style" assume that they're not incorrect, and if there's a lot of feedback like that on a comic that seems to be struggling to build an audience, it's good to assume the readers have potentially identified a problem, or in UX terms, a "pain point" that's making them not have a good experience with the product.

BUT after collecting all this data of what the pain points are from users, you should then not necessarily listen to anything they say about how to fix those issues, and instead seek out somebody with a broader picture of the industry, more experience and more expertise to fix it.
Because if the issue is "I don't like the art style", a reader will often suggest, "So change it to an art style I like more" as the fix, when a better solution might be "host your comic somewhere else" or "market it to people who like this style more", or even "change the cover so it better reflects the style of the internal art, so you don't attract readers expecting something different."

Basically treat all feedback as valid, but be pragmatic about what advice you follow for fixing issues, because an awful lot of it will come from people who either haven't actually got a clue how to make a popular comic, or if they made a popular comic they don't really know how they did it (they just emulated the work of others and it happened to work out), or they don't actually care about helping you achieve your goal, they just want you and others to value what they value.

You also need to be honest about what you want to achieve and choose an appropriate critic too, or the critic can't help you. The number of times somebody comes to me like "Critique please! Do me! Do me!" and I advise them based on the assumption that they would like a comic with more readers (because mine has) or to get featured by Tapas or a Tapas contract (again, I tick both these boxes) or do professional work (yep! That's me!) and then after all that work, they turn around like "Oh, well, if it involves changing things about my work, I don't want a bigger audience or to be popular on Tapas or to do professional work, I think that stuff's for hacks, I just want to express my artistic vision through my comics." It's like.... Don't waste my freaking time, then! Just go and do that! Also putting the people who post in critique threads just to get exposure in a popular thread (or maybe under the delusion the critic will be awed into only saying nice things about how their comic is a hidden, underappreciated gem???) and then get angry because they got criticised on blast because RRRGHHHH! That is always infuriating and very disrespectful to the person who read through a bunch of your comic and formed opinions, gave advice, maybe even linked specific tutorials or did draw-overs. Never do that.

Look at reader feedback. Find the problems people are generally noticing, or common points of confusion about your story (or, if nobody's commenting at all, that might be a problem too, or a big drop off in readership at a certain point). Then try to find a critic who has had a greater degree of success at the thing you're trying to do. Do not expect good criticism from somebody who generally has a low opinion of the sort of thing you want to make. The critic doesn't have to draw or write just like you, but they can't hate the style or genre you want to work in, or their opinions be tinted by that. The best advice will always come from somebody who genuinely wants you to succeed and sees what you're making as valuable and worthwhile and really, truly does want to help you make the best version of what you want to make in order to achieve your goals.

Yeh it's best to find people you trust and know closely so you won't freak out in public or cause drama.... or freak out in general. I got lucky with this one dude who's been helping me out since 2016. Wouldn't be here without him. Australians rock m8.

having both succeeded quite handily and having royally fucked this up myself, my experience largely boils down to 'remember that the person on the other side of the screen is a human being'.

When critiquing in person, it's a lot easier to be respectful, to temper your negative comments with positivity or understanding, and generally be more constructive.
On the internet, it's A: easier for me to accidentally start harping too hard on the art itself and not actually recognize that I'm giving advice to the human being who created it rather than talking to the art itself, and B: Easier for my often ascerbic sense of humor to sound like outright condescension because I rely so much on human elements like tone and body language to help communicate when I'm not being too serious.

I have a literal college degree in comics, specifically, and have been doing this professionally for over a decade. I also have a not-so-subtle love of teaching, mentoring, and helping people to improve. Because of this, I have absolutely fallen into that trap you described above, where I've dived deep into lectures on basic theory like I'm teaching an introductory class. I can't speak to every critique ever given, but for me it's always an unintentional asshole moment: I am just so excited to discuss and explain my theories on why stuff works the way it does that I can completely forget that someone who's actually at the level to be asking this question in the first place probably doesn't need me to be going over the basics like this.

Again, this is one of those things in-person critiques don't have nearly the same issue with, because if I start lecturing on basics, I can tell when someone's bored or already understands what I'm saying through facial expressions (and their ability to interrupt me and tell me I'm preaching to the choir), but on the internet, I end up feeling like I have a podium to start pontificating from way more easily.

And what's worse, I have been on the receiving end of this exact same situation and I know how shitty it feels: I've had someone go over basic muscular construction on me thanks to a tiny anatomy mistake and it felt so unbelievably condescending that I just ignored everything they had to say from then on.

Communication is hard AF, and I have failed at it more than my fair share of times in the past. I feel like it goes double for internet places like this where it's so easy to decouple the human being you're trying to help from the art you necessarily need to break down and disparage in order to make it better.

I, uh... I don't have any specific advice at this that wouldn't just be repeating what other people said. Just that critique is a skill, and being able to give good advice is something you have to practice to get good at. If you give advice and someone gets pissed at you for it, then take note of the way you phrased your criticisms so you can do better next time. Hell, if possible, outright ask other people to critique your critique (it's how I've recognized a bunch of my own faults that I was very defensive of in the past) so you know which things are actually preventing your message from getting across the way you intended.

I'd also add that taking critique is, to a lesser degree, a skill as well. It's something of a two-way street in some situations, and some people just don't want to accept criticism to begin with. Most of us have more experience with getting criticized, critiqued, and/or corrected than the other way around, so there's usually less need for development on this part, but I've found that genuinely good advice given in a bad way can still be useful to me if I adjust my perspective and make an active effort to give the benefit of the doubt.
I mostly added that last bit because it has genuinely helped me to be a less angry/hateful person when it comes to my art. I can be a lot more accepting of people being dicks and not let it affect me nearly as much as I used to by focusing on the fact that they just gave me free art advice, instead of focusing on the fact that they were a shitty teacher.

(Also in regards to your story about the discord server, was it me who did that? That absolutely feels like a 'me' thing to do, but I don't remember specifics well enough to determine if it was me or not.)

I think it's that when we don't know someone at all, it's harder to critique. Creators I've known for a while, I know a bit more what their end goals are, so I can give better advice, or point out things that I know are a problem for them, because I've seen more of their work. (that and I know more of how they'd react, so I know if I should be firmer or go easy)

But also, I just have learned through doing long critiques once upon a time that you don't want to leave too much in a critique. Keeping it short and simple helps them out so much, because even the smallest changes take FOREVER to implement in a comic (like just changing your font size, hell that's going to be exhausting.) So I focus on the big 2-3 things I think stuck out, and I hold my tongue for the rest of it. They wouldn't be able to make a whole page of changes even if they wanted to, especially if they had 100+ pages already written.

I think an element of giving critique that people often aren't aware of is you get better at it with experience, just like anything else. Part of why people who are really good at giving advice that will genuinely improve a work are rare is that you basically need to take a person who is keen to give people advice.... and then give them several years to experience what works and what doesn't...while hoping this doesn't kill their passion for giving advice.

Giving criticism on comics, especially webcomics, where chances are the pages are already published and out there in the world, is hard! There are so many different aspects to a comic; the presentation, the typography, the storytelling, the art... and there are so many panels in any given update it's just not as simple as feedback on one illo. It requires a lot of time and focus to go through a person's comic looking for consistent, recurring issues, trying to decide which are creating the biggest potential barrier to reader interest or enjoyment and then working out what advice to prescribe for the creator that can fix those.

And people often use the idea that "if Criticism isn't phrased nicely it's not worth listening to" to then discount all your hard work. Like, let's imagine the scenario like this:

Person: "Hi! I want to hear your honest opinions about my outfit."
Critic: "Okay... hmm... let's see.... Oh! Your shoelaces are untied, you should tie your shoelaces or you'll trip over!"
Person: "UGH. Okay, well, I'm not listening to your advice because you didn't give any praise to balance your criticism."
Critic: "Wha-?"
Person: "You're SUPPOSED to say one nice thing for every negative thing you say, otherwise you're a bad critic and I'm allowed to not listen to you. And anyway, I didn't want advice about my shoelaces, I think they're irrelevant. I'm really just looking for feedback on the colour of my shirt and pants."
Critic: "Oh... uh... the shirt and pants are fine? But your shoelaces seem to be a much bigger issue-"
Person: "Ah! I said no commenting on my shoelaces! Bye!"
(and then the person tripped over).

It sounds far fetched phrased like this... but honestly it's not far off. It's really not uncommon at all for people to ask for feedback and then to police it based on your tone, or suddenly decide you're not allowed to give feedback on areas they don't want to change or get herded to only talk about areas so small and specific you can't actually criticise them. It's annoying to deal with this sort of "I don't like that you didn't phrase your criticism nicely enough" response because... I'm sorry, you don't think taking up hours of my day to read your comic and make detailed notes and find appropriate suggestions for you so you can hopefully achieve your goals is already a nice thing to have done for you?

This kind of response can really make giving criticism feel like a thankless task, and causes a lot of the people who genuinely want to give good advice to just lose motivation for it after experiencing it a number of times. A lot of these creators really don't seem to have considered the possibility that their comic that nobody's reading could be lacking readers because it's not a very fun thing to read; it might be confusing (poorly laid out, bad font choices) or boring (poor pacing, no stakes or plot direction) or very unpleasant (very nasty or unappealing tone or themes). A lot of people who come asking for criticism could really use a bit of humility and to arrive with the sentiment, "My comic doesn't seem to be very entertaining for people, please help me." instead of coming in like "Here is my comic! As you can see, it is VERY entertaining because my mum says my OCs are the best and I have a very original concept and worked very hard, so hop to it and read it all and then tell me all the secret hacks you used to make people read your comic so they'll read my masterpiece, and also you need to be nice to me while doing it because that's the law of good criticism!"

slightly off-topic, but your comment about webcomics being criticized after publication made me think of this:

From the receiving standpoint, one of the best phrases I've learned to live by is 'I'll fix it on the next page'.

Like... if people give criticism of something I've already made and published, I can run myself ragged doing, re-doing, and re-re-doing the same content over and over, or I can accept that what I made isn't perfect and use the criticism to improve the next thing I do, which is usually the next page of my comic.
Personally, I LOVE finding webcomics that look like garbage in chapter 1 but have been going for 5+ years and look incredible by the end. Watching the artist grow and improve incrementally is as much of an appeal for me as the narrative itself developing and playing out. I personally bank on that in my own work, and accept that I'm gonna have some stinkers.

If someone criticizes my work and tells me any given part of my comic that's already published is bad or I've done something wrong, then holding to that idea of 'fixing it on the next page' helps stop me from despairing over how bad the stuff I've already made is, and instead focus on making the next thing better.

Obviously, this should be used in moderation and with discretion: If my comic is straight unreadable thanks to horrible font choice, terrible layouts, or totally worthless draftsmanship, then maybe it is worth fixing just to make that introductory experience more worthwhile. It's more meant to prevent agonizing over making sure every page is perfect and instead putting that energy towards making your next piece better.