You can apply your discount code during the checkout process. On the delivery options page, look for the "Have a voucher code?" dropdown. Enter your code there and click "Apply." If the code is valid, you'll see the discount applied in your basket. If the code is invalid, a red message will appear stating, "Invalid voucher. Please try again."
Please note: Once an order has been placed a voucher code cannot be applied retrospectively.
We also have a handy "How to video" that you can watch below: