Start: Grab the barbell with a full, overhand grip that's just narrower than shoulder width. Lie on a bench holding the bar over your eyes with straight arms. Your upper arms aren't perpendicular to your torso in this position, but this angle makes the exercise harder for your triceps and easier on your elbows.
Finish: Bend at your elbows as you lower the bar until it almost touches your head. Pause, then lift it back to the starting position. Keep your upper arms in the same position throughout the exercise.
Use a decline bench instead of a flat bench, to increase the demand on your triceps.