Sit on a flat bench and place a loaded bar on the bottom of your thighs. You can place a pad or towel underneath the weight to relieve some of the stress on your thighs. Place some sort of platform underneath your feet to allow you to achieve a full range of motion on the exercise. A block of wood or a stepping platform will work well. Place the balls of your feet on the platform and drive the weight by coming up on your toes as high as you can. Squeeze your calves at the top of the movement and then return them to the starting position by letting your heels down.
Avoid using jerky motions or body momentum and focus on moving the weight with your calves only. Utilize a full range of motion by coming up on your toes as high as possible and dropping your heels down as far as they will go.