Make 3 Pages each with 20 Thumbnails. Just generate basic shapes into tiny compositions. Get an eye for composing each little frame quickly, without effort and as little thought as possible. Start by using the basic shapes of Circle, Square, Triangle etc and branch out if you need more variety. Ask yourself the questions afterword "did I go too slow?" "Which ones do I like the best?" "Do I have a tendency to do more of one type over another?"
If this goes to fast do more. Like hundreds more.
If it takes you an hour it is taking you too long.
Use whatever materials you want, mix them up if you want.
For Inspiration Check out