Take a shoulder-width stance. With your right hand, grasp a single handle using a supinated (underhand; palms facing up) grip, your other hand rests by your waist. You should be standing in a position allowing your performing hand to be at the same level with the pulley. Bend over slightly and bend your elbow to the acute angle, i.e. no more than 80 degrees. Keep your elbow tucked to your side.
Begin to extend your arm (eccentric movement, triceps contraction) while moving your hand all the way to extension with your elbow locked in to the side of your body. Your hand and your torso are at one level. Keep your wrist firm all the way through the motion. Breathing in, go back to the starting position in the same way while flexing your elbow. Breathe out as you extend your arm. Keep your torso upright throughout the exercise. Repeat as required while alternating hands.
Make sure you keep your torso straight without arching your lower back.
Keep your elbows tucked in to your sides all through the motion.
As you start, choose an appropriate resistance to master the technique well.
If possible, perform the exercise in front of the mirror to be able to set the starting position correctly and to watch the whole movement.