Sit down on an incline bench and hold the dumbbells down and to your side with your arms completely relaxed and extended. This is your starting position.
Raise one dumbbell up as far as the natural range of motion allows you and then lower it back down until your arm is almost completely extended. Then do the same with the other side. You should always keep tension in your biceps. Repeat entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't extended your arms completely on the downward portion of the movement (because you need to keep constant tension in your biceps)
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
Focusing on executing the movement is especially important with this exercise. As stated above, you need to do it in a smooth and fluid motion. Jerking the weight in an attempt to lift the dumbbells will not impact your biceps much.
Thus, take a few practice reps to get the hang of the movement if you have never done this exercise before.