1) Ensure the foot is directly in line with the leg. Toes not turned out as is often the case.
2) To get an effective stretch you need more pressure than just stepping backwards so press into a wall or bench with hands or forearms.
3) Roll the heel backwards so the very back of the heel is on the ground and lift the toes just a little.
4) Straighten the knee completely feeling tightness in the muscles above the knee, lifting the knee cap.
Hold the stretch for at least 30 seconds to get effective lengthening through the entire length of the calf muscles.
Note : in the case of Achilles tendonitis this is not suggested as it tends to aggravate the tendon.