Sit on a flat bench with a dumbbell on each hand being held at arms length. The elbows should be close to the torso.
Rotate the palms of the hands so that they are facing your torso. This will be your starting position.
While holding the upper arm stationary, curl the weights and start twisting the wrists once the dumbbells pass your thighs so that the palms of your hands face forward at the end of the movement. Make sure that you contract the biceps as you breathe out and make sure that only the forearms move. Continue the movement until your biceps are fully contracted and the dumbbells are at shoulder level. Hold the contracted position for a second as you squeeze the biceps.
Slowly begin to bring the dumbbells back to the starting position as your breathe in and as you rotate the wrists back to a neutral grip.
Repeat for the recommended amount of repetitions.
There are many possible variations for this movement. For instance, you can perform the exercise sitting down on a bench with or without back support and you can also perform it by alternating arms; first lift the right arm for one repetition, then the left, then the right, etc.