Twist the dial on the bottom of the pen (it will only turn one way) to release the whitening serum into the brush tip. Keep twisting until the tip is saturated. (It may take many turns of the dial during your first use, but won't take as long for later uses.)
With an up-and-down motion, apply a thin layer with the brush to the front surfaces of teeth that are visible when you smile. Apply extra serum to darker areas, and don't apply any serum on the gums.
Replace the cap.
Allow the serum to dissolve naturally on your teeth. Don't eat, drink, or rinse for at least 20 minutes to allow it to work. You don't have to remove anything from your mouth - it's safe to swallow the amount used in one application.
Repeat twice daily for a week to whiten teeth, or once a day to maintain a white smile.