Balance on one leg, holding the other leg straight in front of you.
Squat on your standing leg, slowly getting to the bottom position where your thigh is pressing against your calf.
Extend your leg to get back to the starting position.
If you cannot get into the bottom position without your heel coming off the floor, you might need to work on your Ankle dorsiflexion (in flexibility and strength). You could also perform the exercise on a slope, with your toes pointing down, or place an object under your heel to elevated it, until you are able to perform this exercise on the ground.
Form tips
The non-working leg is straight the whole time and doesn't touch the ground.
The working foot stays flat on the ground.
The working knee moves forwards in line with the toes.