Begin by holding the ball with both hands over your head. Keep your biceps in line with your ears, this way incase if you feel it's gonna slip out of your hands or you're getting tired it will drop behind you. Keep your feet hip-width apart, knees slightly bent and toes facing forward. Try doing a slow, simulated version of the movement during which you keep your back straight, and chest up.
Slam the ball down against the ground in one explosive movement, bend at the waist, keeping your hands outstretched, and ending at your thighs. Make sure that you slam the ball a few inches in front of your feet so the ball doesn't rebound and hit you in the face. I always look at my shoelaces for guidance and the ball will slam right between your feet about 2-3 inches infront. You also don't want the ball to slam too far away from you where you're running after it. It's about keeping control in your core and slamming down in one straight movement. Catch the ball as quickly as possible and back up to starting position.
Add intensity and variety once you mastered the movement. Start by increasing the weight of the ball you're using first, then add a time element by trying to do as many slams as possible in five minutes. Add variety with a one-armed slam, a split squat slam to work those obliques (alternate sides), or slam from a kneeling position to work the core a little more.