This is a progression from a single leg balance and split squats. Make sure you can balance at the top for 1 second before returning to the lunge. The lunge position should end with the knee just above the floor and both legs at 90 degrees.
This exercise can further be progressed into a plyometric exercise but do not do so until a) you have mastered the balance version and b) you have built up your physical activity to include plyometrics by first developing strength.