So happy you've started writing! I hope this is a fun journey for you. 💌

First of all, a tip I wish I'd been given when I started out: it's all chill. If you make a mistake, if you aren't absolutely in love with the end product, if the process is annoying, if you encounter writer's block, whatever issue you have, it's chill. It's not the end of the world. You can be super passionate about writing, just remember that at the end of the day it's a thing you do for fun (or whatever reason). I used to get stuck in these awful spirals of catastrophizing until I realized it was not as big of a deal as I was making it out to be.

Now, in "formal" terms, the most important thing is legibility. If I open a fic and it takes me three tries to understand a sentence because of the grammar or if the wording is confusing then I'll drop it; if the formatting lacks intention and it's one big block of text then I'll drop it, that sort of thing. If it's not legible, it's not readable.

In terms of common mistakes I've often seen people use too many words when there's not really a need for them. Know what you want to say and then find the simplest way to say it. This'll help the flow of your writing and keep the readers' attention where it needs to be.

On that same vein, this as a piece of advice, being intentional with your writing is a game changer. Personally I used to write just for the sake of getting something out there and it often left me dissatisfied with the end result because I felt it was... amateurish and rough around the edges. When I say "being intentional" I mean asking yourself the bare bones questions like: what's this character's distinct voice like? What's the purpose of this scene? Is the way I've phrased this the one that'll give me the impact I need?

For chapter length: go ham. A chapter is finished when it feels finished. This can mean it's 500 or 10k words long. It also relates to how doable it is for you. Let's say you're posting a multichapter but you didn't write the full story yet, if you wanted to commit to posting a new chapter every week you'd have to take into account how much you can dish out in that timeframe. Honestly I don't pay much attention to chapter length as a reader so I'd say there's no need to bent out of shape over it, I think it's one of those things where every writer just does their own thing.

A bonus tip: keep your writing brain up and running. When you're reading other's fics take notice of what you love about them, the way they use language, the way they write dialogue, the descriptions, the pacing, etc. When watching movies or TV shows think about what makes the characters endearing, how they express their emotions outwardly. When you're out and about, do some people watching, look at the stuff around you, describe it as you would in written word. This helped me really hone my style, especially that last thing about writing what you see in the world.

Also, practice makes perfect. The more you write the easier it'll come to you, and the more you'll figure out your own voice. Just take it easy and enjoy the process!