Alchemist Burn Outs was a novel, so I kinda have to do the opposite of a lot of people and like...edit out things when I get ready to make it into a comic rather than write new stuff. I separate chapters into pages, and from those pages highlight what will end up in the comic. I try not to make too, too many panels per page.
After this step, I take the words and stick em into pages on Photoshop--I shuffle the words to see what fits best, and delete even more words at this step, usually. Then, I put in bubbles to make sure I don't crowd em, and then I draw in the panel shapes and the thumbnails.
But for other things that are intended to be comics from the get go, I just make loose scripts rather than books. I'm the only one using them so I already know what everything looks like and I have an idea in my head of what the scenes will be like so I rarely put down cinematography instructions or details about the backgrounds. I save that for concept art brainstorming.