Stand up with your feet shoulder-width apart. Hold the dumbbells to your front with your palms facing towards you (reverse grip). This is your starting position.
Lift one dumbbell up until your forearm is almost completely vertical. Keep the portion of your upper arm from your shoulder to your elbow as stationary as possible. As you bring the dumbbell up, twist the inner side of the dumbbell a little outward. Then lower the weight back down to the original starting position.
Once you perform one complete movement on one side, alternate and perform the same with the other side. Keep on alternating like this until you complete the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't swing the dumbbell up in an attempt to lift the weight - this is entirely counter-productive
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
This is one of the most stringent exercise movements to perform. If you move your upper arm too much you will target a completely different muscle group (your back: lats). Thus, this exercise is perhaps not the best choice for a beginner.
To execute this movement right you will need to do some practice repetitions with a lightweight dumbbell. Once you are confident that you have the movement down, do the exercise with heavier dumbbells.