Stand in front of the cable machine and grasp the handle with an underhand (supinated) grip. The knees should be slightly bent and your arm relaxed along your body.
Keeping your torso fixed, elbow tucked in at your side, and eyes facing forward, slowly curl the weight up as far as possible without letting the elbow move forward excessively; only your forearm should be moving. Squeeze the biceps at the top of the movement, and then slowly lower the weight back to the starting position. Repeat for desired reps and then repeat for the other arm.