Mount wide dip bar with oblique grip (bar diagonal under palm), arms straight with shoulders above hands. Bend knees and hips slightly.
Lower body by bending arms, allowing elbows to flare out to sides. When slight stretch is felt in chest or shoulders, push body up until arms are straight. Repeat.
Also see Triceps Dip.
To decrease difficulty, get assistance from machine or training partner. Also try performing movement between two benches or self assisted 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.