Shift your weight to your left foot, lift your right knee up, and step into a forward lunge. Your 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. Aim to keep your right foot in the air until you drop into your "backward" lunge, without setting it down in the middle.
This counts as one rep. Complete for 10 reps, alternating sides.