Stand up and hold the dumbbells down and to your side in the "hammer" position. This is your starting position.
Lift one dumbbell up until your forearm is almost fully vertical and then lower it back down to your side, but don't extended and don't relax your arm completely. Keep constant tension in your bicep. After you finish with one side, do the same movement with the other side and keep going 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)
Don't swing your arms in an attempt to lift the weight
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
This is a great exercise that is one of the best exercises for your biceps (upper head - see muscle anatomy chart). You should include it in your bicep exercise regimen on a regular basis.
This exercise is most effective when it's done in a slow and controlled manner. You don't want to rush this movement. Once you get used to it, try to constantly increase the dumbbell weight. If your goal is to grow your biceps bigger, then you need to blast them with heavy weights. If you goal is to tone only, then you don't need to focus on increasing the weights too much.