I can act out "condescending". The typical bit would be looking down with a raised head, raised eyebrows, lowered eyelids. Maybe a smirk. There's overlap with other modes and attitudes of course which get fixed by context and dialogue. This would occur when the person is prideful/arrogant and thinks the other person has trouble keeping up with them.

I think you can logic emotions. Sometimes you have to. However, I'm a little... lost on what I should say. But I'm curious.

when you say condescending my inner self thinks BOSS . maybe because i have had a stream of them in the past. sarcastic smug know it all's who think you are only on this planet to make them look bigger than there artificial taste of power deserves. ( sorry that slipped out, deep breath)

when it comes to my characters I know how they all act and speak. I can hear them talking when i write, and change anything that wont sound like them. The best bits for me to write are sarcasm and arguments ( you write what you know)

as for the bonus points i think your a bit shy.

Well, I disagree with about 95% of what my characters do and say, but I can write them (hopefully, convincingly so), because I study them and analyze their behavior to a T. The "trick" is to put them in many different situations, let them act, and learn from each experience. You have to force yourself to bite the bullet, get into their proverbial shoes and learn detachment, so you can observe them better.

Honestly, if I had to agree with my characters at all times, I'd be a war criminal or a drug addict, so... I think you get my point

Get into their shoes. There's a reason why your character is acting condescending, or acting with any emotion. Get behind that reason. Let the hate flow through you...yesss...I mean. Uh...

But for real, your characters came from you; they inherit the ability to feel emotion from you. You can feel their emotions and get into their head better than anyone. Think about their reasons for acting the way they do and it may come more easily. On the flip side, you may find that the wrong emotion is being conveyed and you need to change the emotion you're writing.

Additionally, a lot of emotion comes from facial expression and body language, so if you're doing a comic, faces help.

Since you asked for an example, here's an exchange between two characters where tensions are high and they're generally snarking at each other:

“In case you haven’t noticed, I am a commoner. I’ll wear what I’ve got on, thanks.”
He tossed the package [of clothes] back. Din returned the throw.
“And, in case you haven’t noticed, I just saved your life. I do believe you at least owe me this much.”
“No way. Those clothes would pay for the deed to my entire town. I ain’t wearin’ something so exorbitant.”
“But you are not wearing those rags either.” He caught the deadly look on Conan’s face and changed tactics. “There is a reason beyond aesthetics that I want you to wear these.”
“Are they assassin-repellent?”
“Are they—Wh….No…”
“Then I see no point.”

Sarcasm, using one's own phrase against them, and wordless expressions can all go a long way.

When I was in school and we did a class on character design we first drew out tons (and I mean like 50) thumbnail sketches based off of our research. Example, for the class we were each designing a pro wrestler with a twist. Like mine was called Madam Mortem and her design was based off of Italian plague doctors. Her design turned into something tall and dark and creepy, but we then had to make short one page comics of our characters in certain situations, like going on a blind date, what's their day job, what would they be like on a college campus, what product would they sponsor. Random things.
Also find a character development question sheet and answer them all as honestly as possible.
http://www.freelancewriting.com/articles/2012-character-development.php

This character had a mask on so emoting was very gestural, and of course I didn't feel like I agreed with her on many things she ended up believing (That a great white raven would come down from the stars and smite humanity) but the trick is putting them in all these situations you get to a point that you understand the character. It's kinda like getting to know new people, if you're patient in getting to know them and their depths you can relate to them somehow, or at least understand them, even if you dislike them, you'll know what they'll do and how their emotions will work.

I had my fair share of RP'ed before with all sort of character come in a large variety of personality so I guess it just came naturally for me to write characters with different set of personality than me.

To put it in my opinion... don't try to force your personality on your character and instead of thinking about what YOU would do in those situation, think about what THEY with their own personalities would do. You said you can't write what a condescending characters would do because you don't agree with them- well, first to get over this you just have to "stop" thinking about said character emotionally. Use your logic to figure out what they would do. We all can imagine what a crybaby would do in a haunted house, you might not be a crybaby, but surely you can write such scenario just by using your logic and reasoning.

I used to have a lot of condescending jerks (I mean character) back when I used to RPs, I love RP them jerk, because they let me explore a whole different point of view in life. Often the interaction between them with the other characters belong to other people always come down to not a very pleasing outcome.... LMFAO Condescending people are firm on their belief and often act like they are superior to others. I guess in a way condescending could also be attractive because these type of people are usually very confident, sometimes to the point full of themselves. But because of this, they can also be considered "stubborn" and "arrogant"

For how I do it... well, read story with said personality character, watch movie, anime, do some research. You don't need to know them in real life to know how a condescending person would act, you just gotta observe as many of them as possible.

I think empathy is a really important thing to cultivate for a storyteller.

Like, you may not agree with a character, but on some level you gotta understand where they're coming from -- and not just in the sense of "he acts standoffish because he lost his mother when he was young," a simple cause and effect, but in the sense of, what are they feeling inside?
I don't think it's possible to write a character you don't understand and do it well. If you're just basing a condescending person on what you've seen other condescending people do, it might be accurate enough, but it's gonna ring a little bit hollow. I think you have to learn to imagine being "in their shoes," as respheal said.

I have a character who is very condescending and tends to figuratively shoot himself in the foot a lot by dissing people who could have helped him, which is not how I am at all. I'm of the opinion that you should try to be kind to everyone if you can -- heck, if nothing else, people will be nicer to you if you're nicer to them, and your life will be easier. Whereas this guy tends to be of the opinion that he owes them nothing, and treats people badly until he has reason not to. GEE DUDE, I WONDER WHY NO ONE LIKES YOU.
Dealing with someone with this attitude in real life is exhausting, but for this character I have to be able to understand it. I have to look through his eyes and see being kind for no reason as a really unfair imposition, see that would make him uncomfortably vulnerable, see the emotional walls he has up and a deep fear of being belittled that makes him quick to put others down. His actions to me aren't just "what a condescending person would do," they are a reaction to all of his beliefs and fears and experiences

I mean, it's not just friends -- I've been interacting with people all my life. There are pieces of kids from middle school or high school, or even co-workers I kinda knew, that find their way into characters. You don't have to base your shy character off of A Specific Shy Friend... you just have to have interacted with enough shy people to be able to empathise with that mindset.

The flip side is that, in a way, all of my characters are a piece of me. One character is much more reckless than I am, but he has my sense of protectiveness so I get why he's doing it. Another is shy in a way I'm not, but has a lot of my beliefs about how to treat others. One character tends to say the thing I want to say (but I know better) or does the things I stop myself from doing. All of my characters have a piece of me in them, and in that way, I can relate to all of them somehow, even if I don't always agree with where they take that.

i'm not that good with feeling emotions either, but i think all of the advice on here is really helpful. you said you already know how to explain what they're like and why, so i think you have a good profile of your characters. when it comes to writing their dialogue and conveying their emotions, i would just do it by trial-and-error.

i have a lot of specific scene 'archetypes' floating around in my mind -- i.e. argument between lovers scene, where one is angry out of concern and the other is completely clueless of this. i just insert my characters into that prompt and flesh it out from there. since you asked for specific examples, i'll throw one in here.

A: ...I'm sorry. I'm not trying to make you anxious.
P: I'm not anxious, I'm angry.
A: Why are you tapping your medal then? Kairi only does that when she's anxious.
P: (caught off guard) Well...I'm not Kairi, obviously. It's just a habit.
A: ...Okay, if you say so. But can you please just tell me what you were talking about? There shouldn't be any reason to keep it a secret unless it's something bad.
P: It's --
A: And don't say it's none of my business! I'm an ambassador too.
P: (hesitates) We were just...flirting.
A: ...Flirting?
P: That's what I said, yes.
A: Pfff -- (starts laughing)
P: (flatly) See, this is why I didn't want to say anything.
A: Pfffhaha, I'm sorry. (stiffled laughter) It's just -- very unexpected. Here I thought you were all business.
P: (blushing) I have a life outside of the Guardian's service. Although, it was still...inappropriate of me.
A: (chuckling) Well, I won't tell her about it, if that helps.
P: You better not tell anyone about this.
A: I won't, promise! (grins) Though I guess it's not too surprising -- that dictator reminds me of another white haired god.
P: (blushing more) Shut up.
A: Guess you have a type, hmm? Though Drakon is definitely more muscular than Armonia...can't say I blame you.
P: (buries face in pillow, groaning) I am not having this conversation with you.

the context for this is that P is trying to get away with lying to A, who falls for it. there's obviously more that comes before it, but my goal for this was to express suspicion (A), uncertainty (P), cheekiness (A), and annoyance (P). kind of a back-and-forth sort of thing. sorry if that doesn't help at all lol

Am I right in understanding that your problem is that you can't conceive of how a condescending person would act and think? Or is it that you can conceive of it, you just don't agree with it?

If you're struggling to understand what certain emotions look like then I have the tool for you! It's called the Emotion Thesaurus1.

Here is an example of the kind of thing it has written for 'scorn':

Definition: extreme contempt or derision; regarding as inferior

Physical Signs:

A biting remark
A smirk
A quick, disgusted snort
Crossed arms, a wide stance
A deliberate eyebrow raise and head tilt
Projecting hurtful observations: I'd be embarrassed if I were you.

Internal Sensations:

A puffed up feeling
Adrenaline rush at taking away another's power

Mental responses:

Anger
Superiority
Elation at delivering a blow to an opponent through dialogue or action

Cues of acute scorn:

Gathering like-minded people and encouraging their scorn
Forcing the target into circumstances where he is sure to fail

Cues of suppressed scorn:

Turning away
Shaking the head
A slight muscle jump in the cheek
Clamping one's lips tight shut to keep from saying anything
Making an excuse to leave

That's just a little bit from one emotion in this book, other categories include worry, suspicion, regret, love, hatred, guilt etc.

Highly recommend it for occasions like this.