The tricep dumbbell kickback is a tried and true exercise to isolate your triceps which are the muscles on the back side of your upper arm.

To perform a kickback properly:
Stand to the right of a flat bench. Bend your left knee and rest it on the bench, staying parallel to the bench.  Place your left hand flat on the bench ahead of you for balance.  Keep your back as flat as possible.  Pick up a dumbbell in your right hand and bend your arm to raise your elbow up to shoulder height.  Keep your elbow close to the side of your body.  Do not let it flare out.  Let the dumbbell hang down. Look straight ahead.  Press the dumbbell back toward your feet and toward the ceiling until your forearm is parallel to the floor.  Pause for a moment, squeeze your tricep, and then slowly return to the starting position.  Finish one set, then switch sides and arms.


Tips:
Only your upper arm should move.
Keep your back straight.
Look straight ahead.