Lie on your right side pressed against the ball. Your legs extended out, one in front of the other for stability. Your right arm around the ball for further stability.
Holding the dumbbell in your left hand, resting in front of you on the ball, lift the dumbbell straight up. Keeping your elbow slightly bent.
Hold at the top of the movement for a few seconds, then slowly release back down.
Tips
The key is to get into the right position on the ball, once you are stable you can perform the exercise effectively.