Floor squat

Stay on the floor, on your elbows and knees. The feet should be nearly straight, i.e., not pointing out significantly. Pull yourself down (as if lowering yourself in a squat). Don’t stay their statically, instead move up and down. If you easily get ‘below parallel’ you should widen your legs.

Interestingly, this is a decent way to find a proper width for squat stance for many powerlifters!