Charlie Martin, Late-Blooming Athlete: Week 2 – ‘Quality of Life’ and 'The Un-Calloused Butt Mystery'
Heels never leave the ground, hips move back and down first (don't start by sending the knees forward), feet about shoulder-width apart and pointed out a bit. You’ll eventually be able to experiment with different foot positions. Back stays as straight and stable as when you are standing upright with proper posture. A full squat in terms of exercise is hips below knees. (In the deep squat, more of a resting position, your lumbar curve goes "hollow"; this is not a good position to be lifting weight in.)
Charlie: whatever else you're doing this week, practice your squat, get as many tips as you can about good form, and put it in your workouts however you wish. Let us know if it feels more comfortable at the end of the week. Never touch a Smith Machine, a leg press, hack press, hamstring curl, etc. again. If you are looking to get fit, ignore all the stuff that Brad wants you to do on "leg day", and just focus on having a great squat.
You cannot be fit without one.