Stretch one leg while performing a deep squat with the other, so that your glutes touch the ground and without lifting the heel at any time.
When you are down with your leg completely bent, you rise up onto your toes for a second to contract your calf muscle.
Return to the starting position to complete one repetition.
To make the exercise a bit easier or if you find it hard to maintain balance, you can try doing it on a sloped surface or with some weight in your hands placed in front of your chest.