Click the "BUY NOW" button on http://namecheap.com to add any items you wish to buy.

You will be redirected to your cart. Click "Cart" in the top right corner if you have navigated away from your cart.

In the bottom right corner, click "Enter your voucher code.".

Enter your code and click "APPLY." The discount will be applied to your order immediately if your voucher code is valid.