Begin by kneeling down in front of a ball, leaning on it with your hips and core. Place your hands on either side for stability.
Now, keeping your legs out extended behind you and your hands firmly on the ball, extend your back feeling a slight contraction in your lower back. Keep your chest high.
Hold for a few seconds and then slowly release back. Repeat.
Tips
Do not over-extend. Extend to where you feel comfortable.