Add an item to your cart / go to checkout
Visit the item you want, select it, then “Add to cart” (or “Add to Bag”) and proceed toward checkout.

Look for “Coupon”, “Promo Code”, “Discount Code”, or “Voucher” box
On the cart summary or checkout page, many sites include a field labeled something like “Enter promo code” or “Apply discount”.

Enter your coupon code exactly
Type or paste the code (watch for spaces or capitalization) into that field.

Click “Apply” or similar
After entering the code, click a button such as “Apply,” “Submit,” or “Use” to have it validated.

Check if discount is applied
The order total (or specific product line) should reflect the discount. If not, the code may be invalid or expired, or may have restrictions (minimum order, specific item, etc.).