Create a line on the floor with some tape. Make sure the distance to the wall is approximately two meters. Stand behind the line and take the tennis ball in one hand. Throw the ball underhand against the wall and catch it with the other hand. Now throw the ball against the wall again and catch it with the hand you started with. Try to repeat this as often as possible within 30 seconds.
Also try the test overhand. This makes it a little more difficult.