A tennis ball can be used on almost every part of the body. The easiest way to get a feel for it is by doing foot massage.
1. Stand with the tennis ball under one foot.
2. Feel around until you find a sore spot.
3. Imagine the surface of your foot curving around the surface of the ball as you...
4. Lift up and away from the ground. This is very important! You are not grinding your foot into the ball! You will go much deeper and experience a much fuller release without damaging/irritating your tissues if you let gravity do its work!
5. Hold the position until the pain eases.
6. Find another sore spot on the foot and repeat.
7. Explore the arch, the side, the heal and the individual bones in the ball of the foot. See if you can find spots between the ball of your foot and your toes.
Now with the above in mind try finding points all over your body! Front and back. The back is easiest but try the chest and the quads. Use one leg to act as the pressure for the calf on the other leg. Have fun!