Position
Adjust the seat so that your knee joints are aligned with the axis of the machine. Adjust your leg pad so that it is just above the heels. Lean back against the machine with your shoulder blades and sacrum pressed firmly against the back pad and a natural arch in your lower back. Begin with your knees flexed and the weight just off the stack.
Technique
Contract your hamstrings. Begin pulling your legs back slowly, keeping your lower legs straight and feet relaxed. Continue to pull your lower legs back until they form an approximate 90-degree angle with your upper legs. Hold, continue to contract your hamstrings while stabilizing with your trunk muscles. Slowly bring your legs back out to their original starting position. Repeat.