Select a weight that you're able to use for six to ten reps with strict form.
Sit down and adjust the leg pad (up or down) to make room for your legs. When locked in place, this pad should rest on the midpoint of your thighs.
Stand up and grasp the bar firmly with your palms turned away from your face. When gripping the bar, your hands should be about 3 to 4 feet apart.
Keep your arms straight as you slowly sit down and slide your thighs under the pads.
Pull the bar down very slowly, keeping your torso upright at all times.
Pull the bar down to the top of your head, and briefly pause at this point. This keeps your shoulders and arms from doing more work than your upper back. It also prevents you from moving your torso too far forward or back.
Extend your arms completely (over your head) as you lower the weight.
Repeat until you reach failure (you can't perform any more reps with strict form).
Other Key Points and Reminders
If possible, face a mirror to assess and adjust your technique.
Pull the bar down evenly, keeping it parallel to the floor.
Don't tense your trapezius muscles; keep your shoulders completely relaxed.
Pulling the bar to your chin requires greater use of your shoulders and arms. Pulling the bar to the back of your neck involves these groups even more.