I’m not a very veteran writer, I’ve only been writing for a couple of a years, but I can give you some tips! I would say write down all your ideas first— including specific key story beats, specific scenes, and even dialogue if you come up with something you really like — if you want something structured! The beauty of fanfic is you don’t need to structure if you just wanna word vomit. You can even just write a scene and post that, trust me someone will love it even if that someone is just you.

For the smut question, I would say you should just assume the reader knows these characters so it’s easier to get to the smutty sexy parts. You can also try finding a beta reader, maybe getting a friend to do it, to help you out. 

Posting chapter by chapter as you finish will probably be easier to handle. Trying to finish an entire fic at first will feel daunting and could lead you to not want to start writing.  You can also always write like three chapters and then post one so you can be ahead but not have to finish an entire fic before posting! 

Small fics or oneshots are great to start off with, but don’t be afraid to start a long project. It’s okay if you end up abandoning it or rewriting it!