Concerning writing improvement, there is no super quick or easy way to improve. Once in awhile you’ll have an “aha!” moment, or figure out some rule that had eluded you, and that’ll be an immediate—but minor—change.
When I was a wee writerling on ff.net, I had someone leave a constructive critique on one of my stories. It was all very polite and helpful! But the thing that I recall most about that was something along the lines of “you can’t end dialogue with periods and still have dialogue tags”. (So none of “ha.” he said.) It was something I could do quick and immediately to make my writing! That was great! So be on the lookout for technical rules like that.
But overall, that is finite, and not really improvement. Not the kind authors actually want.
There’s the usual answers—read a metric fuckton, keep writing, experiment—and those are all important things! Definitely read. Read a wide variety of styles, genres, and things. It’ll help you so much.
But what will help you even more is to keep writing, too! Your style evolves over time, no matter how happy or unhappy you are with it, and generally it’s in the “improvement” direction. So just keep chugging along! Nothing will change overnight, aside from those little “aha” moments.
If you really wanna get down in the nitty gritty of it all, though, then here are a few exercises and questions to pose:
Go find your favorite completed book or fanfiction, at least 50k and multi-chaptered. Reread it all in as close to one or two sittings as you can. Think about the story as a whole. Did you pick up on more things this time around (foreshadowing, emotional tone, subplots)? What were passages that stood out to you? What did you feel were the strongest character moments, the strongest plot moments? Were there any lines of dialogue that made you put down the book/fic and just breathe for a moment?
Think about what makes this story your favorite. What story parts do you like, and wish to emulate? How did the story flow at tenser parts, and at quieter parts? Look at sentence structure and word choice.
As an experiment, try to switch up your style! Only for a short story or something. If you actively try for something different, then it’ll be more obvious what your comfort zone is, and you can examine that.
I don’t mean write completely different, but maybe switch tenses for a project, or write in a different genre, or about a different character personality type and let the narrative voice match theirs.
Think about stories/storylines/etc that disappointed you. What was the disappointing part? If you go “hm, I think I could do that better”, what makes you think that?
Think about your favorite author(s). Do you like them for one project/series, or across multiple things? What draws you to them?
Know where and when to break writing rules. A very common one I myself break and see broken the most is fragments. Technically, you shouldn’t have them! But they can serve a purpose, if used well. So know the rules, but also know that the rules aren’t absolute.
Something that (embarrassingly) really helped to improve my writing is step one: have a writer crush. Step two: interact with your writer crush. (Optional step three: find out your writer crush reads your writing.) It can be a huge kind of ego boost, but it can also be something really great to strive for, too. You like their writing (why?), so you can passively learn from them, but you can also ask them questions! Find out from the source why they went with certain ideas, or plot points, or styles, or even POV/tense/etc. It can honestly just be super helpful to ask other writers random shit.
On that note, interact with other writers in general. Network! Befriend! The writing community is a big tangled web, and while it seems cliquey and like everyone knows everyone and not you, it doesn’t have to be that way! Make new friends. Exchange stories, talk out ideas, daydream together. It can be some of the most rewarding non-writing work you’ll ever do.
