I'm enjoying hearing all of your story crafting processes!
I myself let it be based on gut reactions at first. That way I can pick apart what I'm trying to say or how I want to get the audience to feel. The core of the story builds itself off this initial energy and from there I jump around to different stages of the story to give them a stronger foundation. You know, so the story arch makes sense and is satisfying. About this point, I share the idea with someone else so I make sure its heading in a good direction before I get too invested. From that point on there is a lot of filling in the blanks and connecting the dots going on. I then ask myself a variety of relevant questions to better identify the environment and how the people react to it.
I wait a long time to name my characters until I have a solid understanding of who they are. I don't know of anyone else who does this but I reference this book I own called The Secret Language of Relationships to find a matching birthday and how they may interact with different personalities. It's really fun to find out the sociology of people, especially when you guess correctly about how you would expect them to behave with specific characters. I spend a good amount of time trying to determine names. For example, Taylor Pruitt means “to cut” or “Cloth cutter”, her last name means “Brave little one”. In the story she seeks serious change in her life and when she goes out to find it, her hair accidentally gets cut, which symbolizes her emotional and mental transformation.
Anyway, this is about where sketching character fashion is derived from their personality and environment. In Visceral, Zeke's look is a collection of found items from the area he has been stuck for 7 years by himself. His pants show signs of ripping at the heels because he had to grow into them so now they are not as long and baggy on him. His shoes he had to craft because he grew out of his last pair. The list goes on but you get the point.
After all that prep pre production, I finally get to the part where I thumbnail up a bunch of pages and pick them apart till I find the layout and flow I'm looking for. Again, I show it around and get constructive feedback before going into production.
In the end I wind up with documents to keep track of the story outline, specific chapter writing, notes, references, and inspirations. Its a lot of fun!