In animation, you have to think of the movements your characters have and you need to have them on a paper so everything would be easy and clear.
So thinking is the most important factor in doing artwork. Spend time on thinking of and searching for your work; in this case you will have a good result.
In fact Thumb nailing is a visual thinking. You draw what you have in your mind then according to that you make your movie. Sketch simple lines just to remember what you want to do. Draw the key movements and poses. (Keys are the drawings at the beginning and end of every movement or at a change of direction)