The best rocks need to be large enough to paint a design on, ideally about the size of your palm. Choose flat rocks that have a fairly smooth surface; this will make adding details easier.
Make sure to clean the rock before starting to paint it.
Soak the rock in warm, soapy water, and use a scrub brush to remove any dirt or dust. Let the rock dry before you start to paint it.
You can choose absolutely any design you want—cartoon characters, letters, or an intricate pattern. Have a look for some inspiration by just taking a look all around you.