Sit against a wall, with your hips at knee level, a 90 degree angle at the knees, and your back flat against the wall.
Slowly raise your heels to come up on the ball of your feet, then come back down with control.
Form tips
Let your arms hang losely by your side; do not use them to exert pressure on the wall or on your legs (that would decrease the stimulus and make progress harder to track).