Stand in front of the cable machine and grasp the handle with an overhand (pronated, palm facing down) grip, elbows tucked in and bent at a bit less than a 90° angle.
Extend the forearms until your arm is completely stretched. In this position, squeeze your triceps, pause, and then slowly come back to the starting position. Keep the elbows tucked in throughout the movement. Repeat for desired reps and then repeat for the other arm.