In a standing position grab a hold of the dumbbell "hammer" style and hold it down and to your side with your arm relaxed. This is your starting position.
Raise the dumbbell up until your forearm is almost completely vertical and the dumbbell is a few inches away from your shoulder. Then, lower it back down but don't fully extend your arm at the bottom of the movement. You should always keep tension in your biceps for maximum effectiveness.
Repeat entire movement for the desired or prescribed number of repetitions and then switch and do the same with the other arm.
Perform this exercise in a smooth and fluid motion
Keep your upper arm (the portion between your elbow and shoulder) tucked in tight against your body and fixed at all times
Don't extended your arm completely on the downward portion of the movement
Don't swing your arm or body in an attempt to lift the dumbbell easier; this is cheating and does nothing for your biceps
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use a heavier dumbbell.
This is a great exercise for the biceps. This type of grip on dumbbells (hammer) typically extends (elongates) the lower head of the biceps muscle and makes your biceps look fuller and more complete aesthetically.
If you find it difficult to perform this exercise standing up you can sit down on a bench and do it from a seated position.