One of the frustrating (and cool) things about being an artist is that we have our own language. We use words that to describe ideas that only we understand and use.
One such word is thumbnailing. Thumbnailing is a lot like brainstorming, only instead of using words, we use pictures. Sometimes the pictures are well drawn, sometimes they're kind of crappy and nothing more than stick figures. The point of the thumbnail, like a brainstorm, isn't to be pretty but to capture an idea as quickly and clearly as possible. Quickly AND Clearly. Quickly because you need to get it all out before you forget it. And clearly so you can understand it later.
We have a million ideas going through our head at any second, and when it comes to drawing we rarely have a crystal clear idea in our head of exactly what we want. And even when we do, it's good to create a tiny "map" which thumbnailing allows us to check and see if everything works. And because drawing well is a long process, a thumbnail will allow us to keep our mind on the big picture while we get lost creating all the tiny details.
Here are some thumbnails along with the process I used to finish the one I liked the best.
Notice how that while I have all the main ideas I want to capture, from character design to where the camera/viewer is placed, I only do the technical stuff like perspective after I pick a thumbnail to finish (but knowledge of perspective helps make drawing perspective without all the grids and lines much easier)
Also, the thumbnails were done tiny, about the size of my thumb. I made it bigger Photoshop and then cleaned it up.
Here's another thumbnail showing just how rough an idea can be:
And here's the cleaned up version that I eventually rejected because of construction problems as well as the revisions I chose to keep: