Single leg squats using the smith machine. Most people find they can't even do a single one using just the bar. Find a low stool, preferably about a foot high, and use it as your starting point. Find a good spot for your foot in front of it, lower the bar onto your shoulders, push the other leg out straight so it has no contact with floor or any part of your body. Then proceed to stand up, then back down SLOWLY until your butt just barely contacts the seat, then back up. Do not bounce in any way off the seat - that is cheating and uses momentum. I would place the standing foot slightly in front of the bar (dropping an imaginary line from the bar straight down to the floor). If your heel isn't a bit in front of this point you are more likely to pull a hamstring (lowering yourself requires eccentric contraction and with all the force on one leg the hamstring is at risk of a pull the further back your foot is placed). Of course, having your foot placed far in front of the bar makes it easier which isn't your goal.