Sit down on a preacher bench and rest your elbows on the pad. With one arm grab a dumbbell with your palm down (reverse grip) and place your other arm perpendicular to the weighted arm. This is your starting position.
Lift (curl) the dumbbell up all the way until your forearm is almost vertical. Then, lower it back down to the starting position but maintain tension in your arm by avoiding full extension to lockout. Repeat the entire movement for the desired or prescribed number of repetitions and then change sides and do the exercise with your other arm.
Perform this exercise in a smooth and fluid motion
Don't extended your arm(s) completely on the downward portion of the movement
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use a heavier dumbbell.
This exercise requires a strong grip which means it's not the ideal exercise for complete beginners. If you're a beginner, you should do basic forearm exercises such as the Barbell Wrist Curl. After you do easier exercises you can progress to more difficult exercises such as the one shown here.
Keep in mind that getting your forearms in shape will require some time. The good news is that they get stronger fairly quickly as does your grip. Having a strong grip is very important for any kind of weightlifting since all lifting involves gripping barbells, cable handles and dumbbells. The stronger your grip the more control and balance you will have over the weights, especially free weights.