Mount shoulder width dip bar, arms straight with shoulders above hands. Keep hips straight.
Lower body until slight stretch is felt in shoulders. Push body up until arms are straight. Repeat.
Also see Chest Dips.
To decrease difficulty, use assistance from machine or training partner. Also consider Triceps Dip between benches or Self-assisted dips by placing feet on bench or elevation.
Added weight can be placed on dip belt or dumbbell can be placed between ankles to increase intensity.