Sit down on a flat bench or military bench and grab a hold of the dumbbells "hammer" style with your arms extended down and at your sides. This is your starting position.
Lift both dumbbells up as far as natural movement in your biceps and arms will allow you. Then, lower them back down to your sides but don't fully extended your arms straight because you should always keep tension in your biceps for maximum effectiveness.
Finally, 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
Don't swing your arms or body in an attempt to lift the weight 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 heavier dumbbells.
This exercise is very effective for the lower head of the biceps. It should be part of your regular biceps workouts routine (use different variations of it of course). It's an exercise that you can perform standing or seated. The seated version might be easier for beginners though.
When you execute this movement, make sure that you don't cheat - don't swing the dumbbells. If you do so, you will get almost zero effect out of this exercise. Thus, the importance of proper form cannot be emphasized enough.