Initial basic idea or premise, then expand it to have it be the focus of a rough story. (For me, a premise is always what gets me more interested in reading a story rather than knowing what the characters are like.)
From the rough story, develop it further and try to get a whole outline of events. While developing an outline, I try to have an idea of what the characters are like.. sometimes it's basic, other times it's detailed. Have enough to write a script, but leave room for changes. (I find my characters turn out different from their original concepts as I get closer to the end lol)
For the script itself.. I write a first draft, and see what could be changed. Rewrite it at different parts again and again. (Recently, I've just.. expanded every story I was working on like crazy, pushing me to never finishing anything. So I decided to basically rewrite everything once again but with a new condition.) Cut the story down to the essentials, if needed to. Refine it once again.
While writing the script, I'm also doing extensive research on anything the comic might be related to since I prefer to have a somewhat grounded realism to evrything I write. The research done can include anything from the spoken language and location to cultural traditions and governmental structures. There's a chance that much of the research done will hardly be shown, but as long as it can be more authentic, that's fine.
That's my whole process for scriptwriting.. the motivation to actually do it is the hardest part for me sometimes lol