To perform a patch test for a new skincare product, apply the product to a small patch of skin where it is unlikely to accidentally wash or rub away, such as the inside of the arm or bend of the elbow. Apply the product as thickly as you would when using it regularly and leave it on the patch of skin for as long as it would normally be on the skin. If the product is something you would usually wash off, keep the patch on for 5 minutes or as long as the instructions advise. Repeat the patch test twice a day for between 7-10 days, as a reaction may not happen immediately. If your skin reacts to the product, wash it off as soon as possible and stop using it.