The dumbbell overhead tricep extension is one of the best tricep mass builders on the planet. The key to this tricep move is to try to keep your elbows from flaring out on the eccentric portion of the rep, aka when you lower the weight.
At the very start of the exercise make sure to angle the dumbbell back towards the area your spotter would be standing, and not start the move with the weight directly over your head.
Watch the video above carefully to see the starting position and a full set in action. I recommend that you perform 5 sets in the 8-12 rep range and do this move on an arm day or on a push workout day.