HOW: Place the barbell on your back in either a high bar or low bar position (choose whatever is most comfortable to you). Keeping your back relatively neutral/straight, descend in the squat to your desired depth, and then return to your starting position. Keep your core engaged and braced the entire time, remember to breathe!
FEEL: You should feel all the muscles in your legs working.
COMPENSATION: Keep a relatively neutral back during the entire lift.