Sit on weight bench with 2.5kg dumbbell in each hand, forearms resting on thighs so wrists overhang your knees with palms facing down (you may have to lean forward slightly). Bending only at the wrists, lower dumbbells as far as you can, then raise them as high as possilbe. Repeat for one set, then do with palms facing up.
Imagine your forearms are glued to your thighs, to keep your biceps from helping.
