tbh i'm not great at planning for longer fics either HAHA 😭 but if you want to know, the process goes something like this:
1. i get ideas for plot/specific scenes and just write everything down, order doesn't matter. i throw everything into a google doc, separate the scenes with a divider. usually it's snippets (specific dialogues i'd wanna include later) or a few sentences as summary. if it's background/extraneous information, i put it into a section specifically dedicated for that.
2. start on the proper outline. sort everything in order—shuffle around the scenes, move parts around, etc. so it's in the order to be read (not necessarily by timeline, but how i'm going to write it if that makes sense).
3. fill out the middle or any missing parts. and HERE is the hardest part for me lol because i struggle with filling in the gaps between scenes. this usually will take the longeste for me, but yeah, i basically rinse and repeat steps 1-2 (vomit words, add details, then sort) until i have an outline that makes sense.
4. if there's any cuts to be made, i also throw them into a special section for archive or in case i wanna revist it.

overall, the outline will look something like this:
PLOT:
[overall summary - put the entire plot into 3-4 paragraphs, then specifics below]
- act 1
- scene 1 (1-2 sentence summary + any snippets i've dreamed up)
- scene 2
- act 2
- act 3
- etc.
———
BACKGROUND INFORMATION
- character background + motivation -> backstories, how do they develop throughout, etc.
- relationships
- setting
- themes
- anything else that you don't necessarily write down explicitly but want to keep in mind as you're writing

then it's just onto the actual writing itself, the most painful part :)) anyways, hope this was helpful!