Grab a pair of dumbbells. Stand with your feet shoulder width apart. Let the dumbbells hang at arms length on each side of your body. Keep your elbows close to your torso at all times.
Moving only your forearms, use your bicep strength to curl the dumbbells up to shoulder level. Rotate your hands so that your palms are facing upwards at the top. Hold this position for a second to maximize the peak contraction in the biceps. Slowly lower the dumbbells to the starting position. Repeat.
Tips - do not lift excess weight and use momentum to swing the dumbbells up. Use a lighter weight and keep the movement slow and controlled. For variety you can do this exercise with one arm at a time.