Standing a few steps back from the machine and hold the attachment using an underhand grip. If it feels more comfortable you can also stand with one foot in front of the other. Start the movement with the handle at your chin. Drive the weight down to your waist while keeping your elbow at your side at all times. Lock your elbow out at the bottom, squeeze your tricep and then return the handle back to the starting position. Repeat the same motion for the other arm.
One Arm Cable Pushdown Exercise - Start Position
One Arm Cable Pushdown Exercise - Finish Position
Tips: Make sure to keep your elbow at your side at all times in order to keep the tension on your tricep at all times. Also make sure to use a full range of motion by bringing the handle as high as it will go and locking your elbow out at the bottom.