As you bend your elbows rotate your palms TOWARD your shoulder. Keep your elbows as high as you can. Let it stretch in the bottom for a second. Squeeze your hands and rotate them back to neutral on the way up.
Start out lighter on these than a normal DB tricep movement. Don't let your wrists bend when you are in the bottom.
The only thing limiting your sets and reps is going to be wrist strength. I can't imagine going real heavy on these. Still a great movement, just slow your eccentric and pause in the bottom to make it more difficult.