Sit upright on a firm surface or chair with your feet hip width apart.
Sit far enough back so that part of your thighs are supported on the surface.
Place a tennis ball or similar object on the floor in front of you using your weaker arm.
Reach forward to pick up the ball and return to sitting upright.
Reach back down to the floor to place the ball to your left side, before returning to sitting upright again.
Continue reaching to pick up and put down the ball in alternate positions to your left, your middle and your right.
