Apply COSRX Advanced The Vitamin C 23 Serum in the evening on clean, dry skin, right after toner. Place a few drops on the face, avoiding the eye area, then gently pat to absorb. Start 2 to 3 times a week if you have sensitive skin, then increase frequency according to your tolerance. Always finish with a moisturizer, and remember to use sunscreen during the day to enhance the effects of vitamin C.