When I started teaching myself set design I jotted notes (mostly, "Well, don't do that again!"). Those notes eventually mutated into a how-to theater design book, Alice Through the Proscenium, which I plan to e-publish soon.
Meanwhile, here's a snippet from the "Design Process" chapter, one suggested design tool. The quote is from Alice in Wonderland (THE definitive guide to theater):
Brainstorming – Scribble every idea that pops into your head. Fast. No editing. No self-consciousness. Go wild. Only later do you go back to winnow out the very silly ideas… which may be the very best ones.
“You may call it ‘nonsense’ if you like,” she said, “but I’ve heard nonsense, compared with which that would be as sensible as a dictionary!”