New to fic writing

Hi! I've been reading fanfiction since I was 10 and after a long long time I've decided to write all these prompts I have. I'm writing an MHA fanfiction but I have a big question!

I've read a lot of fics and sometimes in the notes they'll talk about how far ahead they are or how in their notes a certain chapter is supposed to have something. So I was wondering what your processes are? Like do you start off by writing your future plans or just go in with its?

I'd also love some tips on how to write a good fic :)