How to do it: Start by standing on top of the chair. You can place the chair next to a wall if you need a little extra stabilization. Dangle one leg over the edge of the chair and slowly perform a one-legged squat with the other leg. Stick your butt out and drive the weight through the heel as you work to get a full range of motion as you squat. Go as low as you can while maintaining balance and form (again use the wall for support if you need to). The objective here is to let one leg do all the work; that means not putting any weight on the foot off the chair. Raise yourself back up just prior to locking out your squatting knee and then repeat. Do 15 repetitions with each leg.
Modifications: To make it easier, keep your hands on the wall for balance and so you don't go so deep. To make it more challenging, try squatting without holding onto anything -- utilizing your core and stabilizer muscles for balance. See if you can get your toes down to the floor!