[John Izzo]

Start with a stationary lunge first. Imagine a line splitting the floor that you are standing on in 2 halves. Keep on foot on the right side, and one foot on the left side. Then descend into your lunge. Play with the distance of the back foot. If you want to challenge your balance, really outstretch your back leg.
Most balance is lost in these exercises because we place our front foot and back foot (elevated or not) in-line with each other, decreases our base. Keep each foot on their own side of the "tape".
Once you 've mastered the balance of that, progress to the Bulgarian split squat.