Most people have forgotten how to squat even though the squat is a natural movement for the body just like walking and running. The right way to squat is with your heels on the ground. This is the only way to lift heavy weights safely without putting an insane stress on the connective tissues of the knees.
If you were to do heavy barbell squats with your heels off the ground and the weight on your toes, like a Hindu squat, your knees would be destroyed in a matter of seconds. The ligaments and the patella tendons would be the most likely victims.
Poor ankle flexibility and body awareness are the most common culprits of this practice.
One of the best ways to fix similar problems and increase the awareness during squats is to perform wall squats.
The wall squat requires the lifter to face a wall from a very short distance. Then, the lifter starts squatting while avoiding touching the wall with his knees and head. If you lift your ankles off the floor during wall squats, your knees will touch the wall.
For a detailed explanation on wall squats, consult the video below recorded by Shapeshifter Media.
In the beginning, the wall squat will feel weird, and you will hit the wall with your head on the way down many times. Keep going as deep as your flexibility allows you to and overtime increase the range of motion (ROM). Before you know it, you will be doing perfect wall squats.
If your ankle and upper back flexibility are really bad, you can add additional stretches.