Step into the squat rack and adjust the pins so that you can easily clear the bar off. Rest the bar on your lower traps and rear delts and lift the weight off the pins. Take a large step back and space your feet about shoulder width apart. With a slight bend in your knees, head up and abs tight, begin by bending forward at the waist and lowering your body toward the ground. Continue to descend until you feel a full stretch in your hamstrings. The distance that you are able to lower yourself will vary from person to person and will depend on body structure and flexibility. Once you have reached the bottom, raise your body back up into the standing position but stop just short of being fully erect. Continue this motion until you have completed your desired reps and then re-rack the bar.
You must maintain good form throughout the entire exercise by keeping your lower back arched, head up and abs tight. Also remember to keep a slight bend in your knees as this will keep the tension on the hamstrings and will minimize stress to your joints. I wouldn't recommend taking this exercise to all-out failure as it will be very difficult to safely re-rack the bar if you get stuck in the bottom position. Simply take the lift to the point where you are one rep short of failure and then stop.