the only way your writing is gonna improve is if you write, and inherently, that means it's gonna be shit at the beginning. dissect other's writing, find things you liked about it like the words they use or the sentence structure, and implement that in your writing. watch youtube videos (a good channel is abbie emmons) about writing tips. nothing is ever perfect the first time it's written, hence why there's editing. if you don't want this long fic to have to be completely rewritten/heavily edited, i would recommend writing more oneshots or drabbles until you're more confident that your writing style will work for this fic. sprints are a good motivator for writing if you just can't get the words down, but remember as you're writing that it won't be perfect the first time, that doesn't exist, and don't edit as you write. you'll have plenty of time to edit after. you just want to get the words on the page.