Thumbnail sketches are very quick, loose drawings. They will help you record what you see and work through ideas without having to worry about making mistakes. Thumbnail sketches are drawn quickly. Somethimes they look a little messy. But they will help you plan your art and remember your ideas.
Thumbnails are usually small.
More than one can fit on a page. Keep a sketchbook handy to make thumbnails and jot down ideas. Try folding a piece of paper in half both ways. After you've drawn your thumbnail sketches, make a more complete sketch that includes the details you like best.