Sit a box to stand on away from anything to grab onto.
Stand on the box with a 5 lb dumbbell or plate in each hand.
Raise one leg up as in the rack squat described above.
Now squat back and down on one leg to the parallel position while moving your weighted hands forward so that in the bottom of the motion they are parallel with the ground. The weight will counter balance you and help keep you from falling off the box.
Return to the start position.
As you squat down be conscious of the position of your squatting knee in relationship to the toe. Do not allow your knee to go forward out past your toe.
Focus on sitting with your hips back and the weight on your heel throughout the entire movement.