I kind of go in stages. I've got a massive whiteboard where I write down the beats and themes I want to happen, then on a Google Doc I'll write out the story beats in broad terms as bullet points like:
- character A goes to place where character B is being chased by police. A decides to help B against their better judgement
- A and B are chased but get away by hiding with character C who is confused. A and C are worried that B will get caught of they let them go now so they agree to keep B safe a while
- Because of this, A becomes a target for the police too as well as B, and they are smoked out in a raid.
Etc...
Really generally describing the events that happen with details of stuff I don't want to forget like where things are happening, any important props that need to be in the scene, how the characters feel about events, etc...
Then I write it out like a play where I only write scene/prop descriptions, character dialogue and didascalies (indications about character expressions and actions), so I have the scene as an actual scene I can work with. That goes through a few edits to get the dialogue and flow up to snuff.
Finally the "play" is split into pages and numbered panels, usually just meaning I stick numbers in front of different parts of my script, but I'll also give indications about camera angles, lenses, blocking, composition, lighting, etc... So I remember when it's time to do the storyboarding.
It goes through so many iterations because I work on the general story alone so I give myself time to really come back to what I've written with fresh eyes and tighten everything up. It also means that I can have basically the entire story from start to finish as bullet points so I know what foreshadowing needs to be done in earlier scenes, what themes need to be established, what character arcs need to be grounded, etc...