What I've found works best for me as far as long fics is to start with a very rough outline of the general idea (not really for chapters, but more for what happens in a specific sequence).
1. Introduce main character, give some backstory
2. Main characters has xyz problem
3. Main character tries to solve problem and fails
4. Main character seeks help from friend/love interest/whoever
5. Some kind of climax event. Then the problem is finally solved
(Can also draw bubbles on a sheet of paper and do the above in that way, with each sentence in a bubble. You can branch out with little side bubbles to add detail)
And then once I know what direction I'm going, I can add more to each part as I think of it.
But I've found that creating character backstories at this point helps me decide what's going to happen in the story. For example, if I decide that one character has a fear of intimacy, that would impact their interactions with others. Or if a character has a habit of lying, has a fear of water, has a dark secret, etc etc. All those things could influence the storyline.
This is the base I use when making a character backstory, which I adapted from the D&D Player's Handbook and various online sources about character building (I'll give a random example character):
Personality traits: Always follows through when he sets his mind to something. Hates being interrupted while he's working. Speaks loudly without meaning to, in a way that makes him seem more intimidating than he means to be.
Ideal(what he lives by/what guides his actions): Hard work is the answer to everything. If something isn't working, work harder.
Bond(what connects the character to people/places): He doesn’t have any close friends, nor any family that lives near him. So he throws himself into his work.
Flaw(can be a bad habit or "negative" trait): Takes on way too much responsibility at work, and won't ask for help or delegate. He's constantly on the verge of burnout.
Goal(what he wants at the beginning of the fic. Can also include an "eventual goal" for what he will want later in the fic): Wants to get a promotion or a raise. Eventually wants to earn the admiration/trust of a certain coworker that just started at his company and has a strange charm.
Paragraph in voice(doing this for each character is pretty helpful, especially if one character tends to use "gonna" and another is more likely to say "going to." Or little details like that: "My favorite part of the day? I'd have to say late evening at the office, once everyone else leaves. It's nice and quiet then. I always get a lot of work done."
Generally speaking, I do this for any character that has lines or is important to the main character in some way. And once that's done, it's much easier to decide how to fill in an outline for actual chapters.
This is the point where I tend to find songs that go with the fic, because that also helps me build the story. Finding at least 3 songs, or a song per character that's in the main story focus, is my preference. I do find more songs later on while writing if those ones stop feeding my imagination.
After all that, I might go from the rough idea sequence above to an actual chapter by chapter outline:
Chapter 1: We meet John Doe. He wakes up alone, follows the same morning routine as always, commutes to work, and avoids his coworkers. People seem slightly uncomfortable around him because he's very gruff towards them. Has one or two coworkers that make an effort to be friendly towards him.
Chapter 2: John Doe is assigned to train the new person at work. He's not happy about the assignment since it's going to get in the way of his own work. That is, until he meets the new coworker. Something about them really draws his attention/or maybe irritates him. They spend the work day together. By the time he goes home for the night, his new coworker is stuck on his mind.
Chapter 3: He has no idea how to balance what he's feeling with how he should act in a professional setting. As the next few weeks pass, he seems to have more and more questions about this new coworker, and very few answers. He realizes wants to know them better in a romantic way. But when he tries to ask coworker out, something interupts or happens to prevent it.
Chapter 4: John Doe doesn't want to ask for help. But then a long-time coworker takes pity on him, and practically forces their help on him. They come up with a game plan.
Chapter 5: John Doe hesitantly puts the plan into action. And as it often happens, things don't go according to plan. But that's okay. Things go even better, in a weird kind of way.
Now obviously, you can do this with as many chapters as you want. Right now I have a 33 chapter fic I've been outlining for 2 months. The document is nearly 18,000 words, and all that it consists of is the character backgrounds (for over 50 characters), a list of what each character does for work in the story(because I need that for easy reference), and then the detailed outline(each chapter has between 3-8 sentences detailing what happens). Whenever I think of more details for a chapter in the outline, I'll go add to it.
And because it's a fantasy au, I've also made maps of the island on which the story takes place and basic blueprints of the buildings in which many scenes happen (doing that also helped me clear up problems with the outline that would have been plot holes, as well as add more details to it. Being able to imagine the setting with visual references really helps).
You can definitely make rough drawings for rooms, or any place for scenes where you feel a visual reference would help keep track of where the characters are/what their body language and action is during the scene. (I once drew a table and 8 chairs, and put character initials on each chair. To remember who was sitting where)
Now once you've nailed down character backstories and your outline, you can do whatever research you feel needs to be done (though you can also save research until you need it. Like if it's for chapter 5, don't look it up until you're writing chapter 5—or even wait until you're editing the rough draft of chapter 5, when the details get filled in)
OPTION 1: Write/finish/post one chapter at a time, using your outline as a guide to move along through the story. This might be more appealing for fics with longer individual chapters or a greater number of chapters. (But for me, it also tends to make me feel anxious about finishing the story if I'm posting a chapter without the rest of the story already drafted)
OPTION 2: Write a draft of ALL the chapters. I suggest accepting that these will be very, very rough. Save the fine details for later and use [Need setting details] or [Funny dialogue] or [They walk from a to b] for places where you get stuck, then just move on and keep writing. You can fill in those [notes to self] later when you're editing.
Then (for option 2) you can either:
2a. Go back and finish one chapter at a time, posting each as you finish it. And what helps me with finishing a draft chapter is to go through each one multiple times looking for different details to add. Ex: Once for dialogue, once for body language, once for setting, etc. It's something I wish I'd have started doing earlier on in my fic writing. And then I highly suggest reading all dialogue out loud during the final read through. I even change my voice for each character (don't picture me doing that. It's kinda embarrassing haha). But as long as you read it out loud in whatever voice you want, you'll be able to tell if the dialogue feels natural. (Looking at a picture of that character before you read their dialogue can help you imagine if they would say it like that)
2b. Go back through the entire fic chapter by chapter, polishing it until you feel like all it needs is a final read through. Then post chapter by chapter as you finish each final read through. (But this method tends to be something more easily done with shorter fics—at least, that's how it is for me)
One last thought I'll leave here is that just because something works for someone else, doesn't mean it will work for you. All you can do it try out various methods until you find what does work for you. (It took me over 40 fics to figure out the methods above, and that was with lots of trial and error. And I'm sure I've got a lot more to learn in the future—which is both daunting and exciting)
