Adjust the machine so that your knee is in line with the rotating cam and get into position. Some standing leg curl machines will provide a resting place for your opposite leg (as seen above), while on others you'll be planting your foot on the ground.
Place the pad on your lower leg just above your foot, grab onto the handles for support, and then curl the weight up as high as you can.
Pause briefly, and lower the weight back down until your leg is just short of being fully extended, and repeat.