I typically start any idea with what I call a "dump" phase. I just start writing shit down on a word document and letting any ideas about the story come out. And I mean anything. There's no structure to it it's just a mess of ideas, dialogue I might wanna include, character names or traits.
From there I move to a rough outline, trying to work the dump into a useable story.
I make a VERY general outline for the entire story then I start writing.
With RK I didn't outline the entire thing just cause the branching narrative would of had me outlining for a year.
Instead I created a general outline of the common threads that I want to tie them all together and work from that to write the branches.