Thumbnail sketches are a simple and quick way to get thinking, start planning your shot and help solve problems early on. Sketching can really help you to be inspired and get creative. Even if you don't draw well, crude thumbnail sketches can help you plan your poses and timing. The extra time spent on planning your work will be worth it in the long run. 

There is no one way to develop a design,  it can be done on paper, digital sketching on a computer or tablet, inspired  by Rorschach  images, with Legos, pipe cleaners, or even pasta.   The most important thing is to really explore design possibilities.  My dad would always say that "your first idea is a cliché or a the first idea anyone would design".  This stage is important and should not be skipped.

Create at least 20-50 well developed thumbnail sketches.  Here are some thumbnail examples, some are more elaborate than others.   Don’t be intimidated by the examples, these are just supposed to be light sketches.  Play with design implementing the 'Elements of Art', line, shape, form, space, color, and texture.  Each sketch should be quick and no longer than a minute or two.  You are just visually exploring directions.

Create at least 10-20 well developed 3d sketches.  Here are some thumbnail examples, some are more elaborate than others.   Don’t be intimidated by the examples, these are just supposed to be light sketches.  Play with design implementing the 'Elements of Art', line, shape, form, space, color, and texture.  Each sketch should be quick and no longer than a minute or two.  You are just visually exploring directions.