Set-up: Set up on two benches (or chairs or other sturdy objects), placing your back on one and your feet on the other.
Lift: Pressing through your heels and firing your glutes, lift your hips off the floor. Your weight should rest on your heels and your upper back - not your toes or your neck and cervical spine. Extend your hips until they form a straight line with your knees and shoulders. At the top of the movement, raise one foot into the air and hold briefly. Now switch and raise the other foot. Return to the bottom position and repeat.
Emphasis: Make sure the end range of motion comes from the hips - squeezing the glutes together at the top of the movement - and not from lumbar hyperextension.