To begin the exercise, lie flat on the floor with both heels on the gym ball and your arms by your side.
Lift your bottom up from the floor by pushing through your heels.
Now keep the heel of the leg to be exercised on the ball, whilst lifting the heel of the other side clear of the ball.
As demonstrated, as you roll the ball towards you, and then away from you, try to keep a straight line between your shoulders, hip and knee.