I write my scripts in roughly the format I expect the comic to be in, with the dialogue for each page and rough descriptions of each of the panels. Usually the panel descriptions are pretty vague since I'm the one drawing them, and I have an easier time figuring out the details while I'm doing the thumbnailing. I also tend to tweak a lot of things when I get to the drawing stage, haha.
For example, here's the script for one of my comic pages:
((scene change! A school bell rings))
NARRATION: Earlier that day…
BRIIIIING!
((CHISHIKI stuffs a bunch of of books into her backpack, Maybe instead of her awkward thoughts here, we see a report card or something confirming her straight A-ness? Maybe both of those things?))
TEACHER (O.S.): Alright, everyone, class is dismissed! And don't forget, there's a test tomorrow, so be sure to study.
CHISHIKI (thinking): Test tomorrow. Right.
((she walks out of the classroom, carrying some books that didn't fit in her backpack))
CHISHIKI (thinking): Maybe I'll stop by the library later so I can study…
((CHISHIKI steps out into the hall, where lots of students are leaving class. Maybe some banners on the wall to indicate this is Taishoki High School))
...versus the final product:
But yeah, I definitely don't think there's a strict requirement for super detailed comic scripts before you start drawing. I say just do whatever you feel works best for you.
(Edit: So apparently spoiler boxes don't work anymore lol)