On a desktop/laptop
Once you are through to the checkout page, there will be somewhere to enter your code on the right hand side of the page. Enter your code into the box and then hit apply.
On a phone or other mobile device
Once you go through to the checkout page, you will be shown the following page. If you click "Show order & apply discount", you will then be presented with a box where you are able to enter your discount code:
Enter your code in the box, and then click the arrow to apply: