HOW: Get set-up in front of a wall or an object to use for balance if necessary. While keeping your glutes and quads tight to keep your knees straight, lift your heels off the ground in a slow and controlled manner, then slowly lower your heels back down to the ground. Repeat
FEEL: You should feel your calves getting a workout, but also your glutes and quads working as well to help keep your legs straight.
COMPENSATION: Do not let your knees bend as that is a different exercise. Do not shift your weight side to side. Make sure you go through your full range of motion at your ankle.