I guess bottomline is that you need to find your own creative process that works best and you can only find it through doing it. Once you you have done a few chapters you'll probably have refined or changed the process completely. I have done so. Each new chapter I change something, and sometimes the process needs to adapt to the story.
My first chapter had very little dialogue, so I hardly script anything. My third chapter had to do with background setting, so I picked a poem I wrote (as a song) and some old chronicles.
I keep writing down ideas in a semi-ordered big timeline, from which I extract the story I am telling. This is good to keep a consistent big-picture background when I don't like to bring it all into the story.
I love writing narrative text, so for the chapter I'm working on I actually wrote the "argument" in novel-style first of all, to have a clear story to tell first. Then comes the process of scripting that into pages and dialogues. I don't want to have an annoying narrator voice, I think that drives attention away from the art details, so I sometimes have to invert my story and use images from other events to build up knowledge for the reader so I can bring tension, confusion, deep thoughts...
But yeah, do the thumbs, they are very important!