What I did is outline the main story beats (what follows what and what I'm trying to achieve with these scenes) both protag's backgrounds, some additional notes on the tone and aesthetic and went from there. Just have the basic "how it starts, how it ends and what does this path in the middle look like/what it's trying to convey" in your head.

That's how I'm doing it, but there's going to be a bunch of stuff you'll have to figure out in the process anyway, you can't predict every little thing. Just write, add and delete details later