Me! I am doing this thing all on my lonesome.
Writing skills are one of those things that are a bit harder to measure than art-skills. Anyone can tell if you've drawn a nose too big, or if your trees are the wrong colour - but it's harder to spot when your character development is lacking, or when the plot isn't quite coming together.
But, like all things, it gets better with practise.
I've been writing for years. Before it clicked in my head that hey, I could make comics (it took a while, for some reason), I wrote prose. I actually did the NaNoWriMo-challenge 10 times, and hit the 50k-word target every time. My first efforts were terrible, but over the years they built slowly towards being less crappy, to being decent, to actually being pretty good - but hardly any of them got finished.
In those 10 years, I wrote a lot, even in between the NaNoWriMo-challenges, and I tried so many things. NaNo was basically my way of throwing stuff at the wall to see what stuck, figuring out my own creative process, building words, learning how to craft plots and characters that didn't fall apart (by crafting a bunch that inevitably did; gotta do it wrong until you get it right). I tried every genre that appealed to me, I practised world-building - I even invented a bunch of worlds and characters that I still keep around, though "cannibalized" their stories - throwing out all the bad stuff and keeping the things that appealed to me. I've got a steampunk-ish setting populated with ideas and characters from a bunch of these old stories, but who play new parts.
What works for me isn't necessarily what works for you, but here are some tricks I've picked up to help me write better:
Plan ahead, but allow for changes of that plan
Some writers need to plan everything rigidly, some writers need to write to find out what happens, building things more organically as they go along. I'm somewhere in between. I need to know where I'm going, but I also need enough vagueness in that plan to allow me to tweak and change things as I realise better ways to put it all together.
Having the plan is essential, though, because if I know where I'm going, I can build up to it better. I can hint at future events, I can plant things in the background, I can drop hints at backstories that aren't fully revealed until later, I can build relationships between characters that impact future events, etc.
Your characters are people, not a list of traits
You see a lot of questionnaires floating around writers' forums. You know the kind - personality tests, but for your characters? What's their favourite colour, how tall are they, name one flaw they have, what's their favourite childhood memory, etc., etc. Heck, I just found an old one in one of my NaNoWriMo-folders that include questions like: "If your character was a criminal, what kind of criminal would they be?" and "If your character was raped, how would they react?" (seriously who wrote this questionnaire). Answering both of those requires thinking about your character, yes, but unless your character is a.) going to be a criminal, or b.) will ge raped during the course of the story, the answers are useless.
All they really give you is a list of traits, quirks, what-ifs and free-floating flaws that aren't really attached to anything, or anchored in anything specific. So you know your main character's favourite childhood memory - does it matter? Will you ever use it in the story? Very likely, the answer is no. I find it much easier to craft a solid, believable character by looking at them and going "Okay, so how do they fit into the story? Why are they the way they are, and what do they want?".
Basically, don't invent a bucket full of character traits that won't matter to the story. Push your characters until you figure out how they work, and do so in connection with the story. Their taste in literature doesn't matter much if the story is about them fighting a dragon, you know?
On a similar note...
Give your characters something to care about
You want these people to do something, and they're going to need a motivation for it. Give them something - doesn't matter how large or how small, but it should have something to do with the story or the other characters - and make them care about it. Maybe they have a lifelong dream of adventure and discovering new places - cool, now you've got an adventurer who isn't going to stay home, or just travel to places people have already been. Maybe they have a friend who would really rather stay home, but they care about their friend too much to let them run off into the world on their own. Now you've got a companion for your main character, who is likely to put themselves at risk to make sure your hero is safe! ... Basically, you've got Samwise Gamgee, but there are worse places to start than that!
Give your characters something to care about, and for extra points, try to align it to come into conflict with what other characters want. Maybe there's a third character here whose own goals put the adventuring protagonist at risk - that's going to put them in conflict with our Samwise-character! I see a confrontation coming on, and whoops, that's a piece of story right there!
Writing is largely re-writing
Your first draft, or your very first outline, probably isn't going to be awesome. That's okay. Go away for a bit and do something else, and when you come back, read through it and figure out the flaws. Try to fix them. Re-write the bits that came out wrong, poke at the corners you wrote yourself into and figure out a way back out of them - or how to avoid them in the first place.
This is why I have an outline with enough vagueness in it to allow me to change my mind. I've already tweaked it a whole bunch, because partway through, I realised some characters needed more screen-time, and some conversations needed to happen that hadn't; I needed to tie up certain threads that were just sort of hanging there.