Put weight in the left foot, lift your right knee up, and step into a forward lunge. The front knee should be at a 90-degree angle with the left knee just barely hovering above the ground.
Push into your right heel, come to stand, and immediately step the right foot behind you into a reverse lunge with the left knee at a 90-degree angle.
This counts as one rep. Repeat with the other leg.
Keep alternating for 40 seconds. Take a 20-second rest.