What are Thumbnail sketches?
Thumbnails are basically shorthand for artists. Quick drawings made with no corrections or details.
They are used to visualize the look of a work of art as well as create an interesting composition. These are usually done quickly no bigger than a couple inches in any direction. It helps gather thoughts and ideas.
What they are not.
A thumbnail sketch is not a detailed drawing, nor is it the same size as the final work of art.
Keep these things in mind when getting started on thumbnail sketching.
Use thumbnail sketches to explore ideas
Think about proportion and composition
Make sure you work fast and don't invest too much time in each one
Keep them simple, no details just shapes, values and forms