My pre-production process is almost all internal. If I don't have an immediate, flash "oh, I know exactly what I'm going to do" reaction to a prompt, I'll spend a day or two letting it simmer on the back burner of my brain, and not trying to force anything. I think a good approach to repeating prompts (like Inktober 52) is to pick an overarching theme or project (for example, each Inktober drawing for me is an animal character solving problems using some technology of some sort. That will keep you from feeling like you have to reinvent the wheel every day or week. If I still feel stuck after thinking for a bit, I like to talk out ideas with a friend or my kids.
Once I select the direction for a prompt, that's when I pick up the pencil or pen. If it's for just character drawing (like I do Inktober), I might do one rough sketch, but I usually just jump to the final sketch, while having some reference images up on my computer. For a bigger prompt or client project, I'll write a brief synopsis of how I see the illustration, including what tools I'll use and what style will best fit it. Then, I'll do 1-3 thumbnails of about storyboard size and quality so that I know where everything needs to go on the artboard in terms of proportion and spacing. I don't want to overwork or overplan too much in the thumbnail/rough sketch phase, so that when I move to the full scale, final sketch, there's still room for creation and improvization.
