Take a big step backward with your right foot, lowering into a lunge, driving your right arm forward to help maintain your balance. Aim to get your front thigh parallel to the floor, with your back knee hovering just off the ground.
Push into your left foot and drive your right knee up, coming to stand on your left leg with your right leg in a high knee. Simultaneously drive your left arm forward to help maintain your balance.
If it's too hard to come into the knee drive from the lunge, step your left foot in to meet your right, then raise your left knee up. That's one rep.