Always work your weakest tricep first. In most cases this will be the left.
Keep the rep timing slow and control the weight. Dont allow the weight to control you on the way down.
Dont lock your elbows out at the top of the movement as this takes the tension off your tricep.
Focus on only allowing your forearm to move, keeping your upper arm as still as possible.
Instructions
Setup for the exercise by placing a bench in front of a pulley cable machine, attaching a single grip handle to the low pulley cable and selecting the weight you want to use on the stack.
Grasp the handle with your left hand, raise your left arm above your head, and turn away from the machine. Sit down on the bench facing away from the pulley.
Your palm should be facing backwards, towards the cable machine. This is the starting position for the exercise.
Keeping your body fixed, your back straight and bending at the elbow only, slowly lower the weight back behind your body.
Pause, and then raise the weight, squeezing the tricep at the top of the movement.
Do not lock your elbow out, and then lower weight and repeat.