Buddy, if I knew I'd tell you. My brain has been keeping me up until 3am and making me write entirely new fics in a different fandom from the longfic I'm so close to having 5 new chapters for. I just need to do some connecting scenes I can't focus on.
Some people find it helpful to make outlines of every scene they want in a fic so they have the framework to jump around in making it as inspiration strikes, some people have fic-writing playlists that may even be specific to the vibe they're trying to capture. Meanwhile I have my brain singing showtunes to me several days in a row and offering me outlines for fics in completely different genres.
The only thing that consistently works for me is making notes in a reminder app when I have scene ideas, and folding them all in later. This is apparently completely bizarre to most of my friends.