Watch the above video and/or follow these instructions:
Start standing up with your feet hip-width apart.
This is a great exercise for developing a good starting position for other exercises like squats as it mobilises the ankle capsule, gets your gluts switched on, your knees and hips in good neutral positions.
Keeping your big toe knuckle firmly anchored to the floor, turn out from the hip so your arch lifts away from the floor but your big toe and heel do not move. Then turn it back the other way.
Do 10 repetitions of this exercise for each foot.