Start with You legs straight, NOT hyper extended. Take a deep breath, and blow out as you raise the heels toward your butt. Inhale as you lower. Make sure you keep your pelvis down on the bench when you lift. This picture is actually incorrect; see how the pelvis is lifted? This is what not to do. Start with ½ your body weight, and do 20 reps. In each successive set, you will increase your weight by the smallest amount possible, and go to down by 2 reps each time.
Back Extensions On The Ball:
Lay on top of the ball, belly button at the highest point on the ball. Hands are behind your head. Feet in the corner where the wall meets the floor, toes out. Lightly squeeze the ball with your inner thighs, and take a deep breath. Lift up until your spine it totally straight, exhaling as you lift. Inhale as you lower. Do 20 reps this and every subsequent set this workout. To make this an even more complex exercise, add rotation to this one.