This is a compound exercise that primarily trains chest muscles, with the shoulders, lats, and triceps targeted as secondary muscles.  Moving the weights from over and behind your head over your chest helps build muscle volume and strength.   You will need a straight bench to lie on, as well as a dumbbell that you can safely push in an arc over your head.  The dumbbell that you are using should be solid, meaning no removable parts.  Using a dumbbell with removable plates will only pose risk of injury when the plates fall apart.

Start by lying flat on the bench.  The occipital part of your head must be resting at the edge of the bench, while the rest of your head is off the edge.  Your hips should be free from the bench as well.  Plant your feet squarely on the foot rest with your toes pointing forward. If the bench you are using is not equipped with a foot rest, plant your feet on firmly on the floor at the sides of the bench, maintaining a 90 degree angle on your knees.

Grasp the dumbbell by crossing your hands placed in one of the inner side of the dumbbell.  Your thumbs act as a stabilizer for the bar, while your fingers hold the heaviest part of the dumbbell.  With a bend on your arms, hold the dumbbell over your chest.  Maintain this as your starting position.

Keeping your arms straight and your elbows locked inhale as you lower the dumbbell over your head.  You must create an arc with this motion.  Your hands should be way over your head, your hands lower than your head, and the free side of the dumbbell hanging over the floor.  You know you are doing the proper execution when you feel your chest muscles stretching.

Hold the position for a second, then exhale as you bring your arms back to starting position.  Maintain the slight angle of your elbows, with your arms straight.  Be sure that the dumbbell is over your chest as your end position.  Repeat with the prescribed number of sets.

If you are new to this exercise, have a spotter hand you the dumbbell and see if you are doing the correct form throughout the entire set.  If you cannot find a solid dumbbell, make sure that the plates are secure and will not fall off on your face as you do the exercise.  A barbell or an ez-bar can be used to intensify the training.

If you would like a workout that explains some great dumbbell work in detail, than take a look at the P90x extreme workout, and Chalean Extreme