The overhead squat is a difficult movement, so you may want to start with a bodyweight squat with your hands out in front of you to get used to the basic squatting pattern.
When trying to perform an overhead squat, a lot of people either shift their weight too far forward onto their toes or back onto their heels, among other mistakes. But you want to aim to keep your weight balanced over the middle of your arches, with your arms over your feet.
Begin in a standing position, holding a dowel overhead with your arms straight and your feet about shoulder-width apart. Initiate the movement with your hips, squatting back and down until the tops of your thighs are at least parallel to the floor. As you go, think about pulling the dowel apart with your hands. Push through your hips to return to a standing position.
Give it a shot, paying attention to your center of mass. Have a friend watch to see if you noticeably shift forward onto your toes or back onto your heels. There are a number of things you can do that may help you stay balanced and do it right.
For example: If you tend to shift too far back onto your heels, do it slower. This may sound overly simple, but slowing down--aim for a count of ten from start to finish--will help you stay conscious of keeping your weight centered over the middle of your foot, with your toes on the ground. Once you can feel the difference when your weight is centered, gradually return to a regular tempo.
If you tend to shift too far forward onto your toes, place your heels on an elevated surface. This will place your center of mass forward, which should help you learn to use your hips more, pushing your hips back and down to counterbalance the forward weight shift. Using your hips in this scenario will come naturally, since it helps keep you from falling forward. You might start by squatting with your heels on a 2x6 board and gradually progress down in height (to a 1x6, folded towel, etc.).
For a demonstration of the overhead squat, watch this video.
--Anthony Slater, Performance Specialist, Core Performance Center