Hold a barbell using an overhand grip, with your hands about 6-8 inches apart. You can also do this exercise with an EZ curl bar or on an incline bench.
Now sit on a bench and lift the bar straight up above your head, with your arms locked out. Having your elbows fixed and near your head, lower the weight down in a full range of motion behind your head until your triceps get a complete stretch.
From this stretched position, pull the barbell over your head using the smallest circle possible and bring it to your upper chest. With your elbows close in, press the weight back up overhead to full extension. While at the top squeeze your triceps for a peak contraction. You have just completed one rep.