Sit down on a flat bench or military bench and hold the dumbbells down and to your side "hammer" style. This is your starting position.
Lift one dumbbell up until it almost touches your shoulder. Then, lower it back down until your arm is almost fully extended (keep constant tension, don't fully extend). Then do the same with the other side. Repeat entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't extended your arm(s) completely when you lower the dumbbell(s)
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
This is one of the best exercises one can do for the biceps. It can be done sitting or standing, although we like the seated position more because it minimizes the amount of "swing" you put into lifting the dumbbells. The key to this exercise is doing it at a moderate tempo. Getting a good squeeze at the top and nice extension at the bottom of the movement will have the optimal, and therefore most desired, effect on your biceps.
If you are a beginner, that's fine, this exercise is good for you too. Just make sure to keep the weight low for the first few months before you decide to try with heavier dumbbells.