Sit down on a flat or military bench and grab a hold of the dumbbell with your palms up and your arm down and to your side. This is your starting position.
Raise the dumbbell up until your forearm is almost fully vertical and then lower it back down but don't completely extend your arm (you should always maintain tension in the biceps for maximum effectiveness).
Repeat the entire movement for the desired or prescribed number of repetitions and then switch and do the exercise with your other arm.
Perform this exercise in a smooth and fluid motion
Don't extended your arm completely on the downward portion of the movement
Don't swing your arm in an attempt to lift the dumbbell; 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. The seated position makes it easy and comfortable to execute. Keep in mind that bracing yourself with your other arm is important. By doing this you will offset the slight pull you will feel from the arm that has the dumbbell. Thus, getting into the right position and providing just enough counter-balance with your other arm is essential to performing this movement correctly.
Another thing to point out is to make sure your upper body is not leaning forward to much or that your are not hunched over. You need to make sure your back stays in a relatively upright position throughout the execution of the entire exercise.