Use underhand grip (supinated) with single-grip attachment, working one arm at a time. Keep arms close to side
and bend fully at elbow. Fully extend and squeeze at bottom. Don't raise arms beyond 90 degrees at top of the movement.
Reverse Grip Cable Tricep Press-Down - Start Position
Reverse Grip Cable Tricep Press-Down - Finish Position
Reverse Grip Cable Press-Down Tricep Exercise Video