Draw a mouth on the tennis ball with a marker, and use the scissors or craft knife to cut along the line, creating a mouth that opens and closes. If you draw the line along an existing gray line on the tennis ball, it will be easier to cut. Just be sure not to make the mouth so large that the ball will split in two.
Create the puppet's face any way you like. Attach some googly-eyes, or use things you have around the house, like paper, felt, scraps of fabric or pipe cleaners to create its features.
Give your puppet some eyes, hair, teeth, freckles or ears, using the markers to draw some of the features.
Allow any paint or glue to dry, and your puppet is ready to use. Just give the ball a squeeze to make your puppet talk.