Hold a stability ball against a wall, placed in the small of your back.
Lean firmly back against the ball.
Walk your feet forwards, keeping your toes pointing ahead and your legs hips width apart.
Place a small ball between your knees and hold it there by applying some gentle pressure inwards with your thighs.
Squat down, dropping your hips towards the floor and bending your knees until they reach 90 degrees.
Stand back up straight, driving the movement with your buttock muscles, and keeping the pressure on the ball between your knees.
Make sure your body remains straight and knees travel directly forwards over your toes throughout this exercise.