Begin by positioning yourself so that a barbell is resting across the front of your shoulders with your arms crossed and elevated so that your elbows are level with your shoulders. Stand with your feet hip width apart, toes slightly turned out. Brace your core and keep your chest up. Bend first at the knees then at the hips, driving the hips back and down, inhaling as you do so. Lower slowly with control until your thighs are parallel to the floor or deeper depending on your range, making sure your knees are pointing in the same direction as your feet and that your lower back doesn't collapse. Return to the start position by extending at the knees and driving the hips forward, exhaling as you do so. Repeat.