Squat all the way down until your thighs are pressed against your calves.
Extend your legs back to the starting position.
Form tips
This will be difficult (or impossible) to execute if your ankle flexibility is too limited. If that's the case, you can still perform the exercise by placing your heels on a slightly raised surface (like a weight plate) or standing on a slope, with your toes down the slope. In parallel, try to work on your Ankle dorsiflexion .