Coupon redemption is done at the very end of the checkout process.
Make sure you have added the discounted item to your cart, then proceed to checkout.
At checkout, you need to sign in, provide your shipping and billing info, gift options, etc.
Then you can enter your coupon.
At the last step, when placing your order, you will see a field to paste your coupon.
Simply right click in the coupon field, and select “Paste”.
Make sure to hit “Apply” and Amazon will automatically update your price.
Make sure everything is correct and “Place your order”.
You have now successfully redeemed your coupon!