Start in a standing position. Hold the med ball with two hands under the chin. Squad downward. A good squat goes so low as to drop the hips below the knees. At the bottom of the squat the upper legs are slightly below horizontal.
Another way to visualize the depth of the squat is to think of a marble. If placed on the middle of the thigh (upper leg) at the bottom of the squat, it would roll back to the and not to the knee.