Vitamin C serum is a skincare product that can help improve the health and appearance of your skin. To use it, first, cleanse your face and pat it dry. Then, apply a small amount of the serum to your face and neck, avoiding the eye area. Gently massage the serum into your skin until it is fully absorbed. Follow up with your regular moisturizer. It is typically recommended to use Vitamin C serum once or twice daily for best results.