Patch testing is a technique that helps you identify what ingredients might irritate your skin. Here's how to execute a patch test: (1) Start by applying the product to a quarter-sized patch of skin on your elbow bend or arm. (2) Leave the product on the skin for 24 hours when patch testing moisturizers and serums, and 1 minute for cleansers. (3) Keep an eye out for any signs of irritation like itching, burning, redness, and bumps. (4) If your skin reacts to the product, wash it off immediately and stop using it. Bonus tip: skin reactions may not occur immediately. To be thorough, repeat the patch test for a couple of days before using new products.