Sit down on a preacher bench and rest your elbow comfortably on the pad. Grab a hold of the dumbbell and hold it up "hammer" style. This is your starting position.
Lower the dumbbell down until your arm is almost fully extended and then raise it back up until your forearm is almost completely vertical. Once you repeat the entire movement for the desired or prescribed number of repetitions, switch and do the same with your other arm.
Perform this exercise in a smooth and fluid motion
Don't extended your arms completely on the downward portion of the movement; you should always maintain tension in your bicep for maximum effectiveness
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use a heavier dumbbell.