Enter it in the discount field at checkout, on the same step where you fill in your shipping and billing details. The total updates before you pay, so you can see the code has been applied.
Two rules decide whether it is accepted:
Exclusions vary by code: the welcome code, for example, does not apply to items already on sale or to bundles, while some seasonal codes do. Only one discount applies per order, so a code will not stack on top of a sale price — and checkout tells you either way before you pay.
Codes run for a limited time, and an expired one is refused at checkout.
If the code is refused on a full-price item, there is more in "Why is my promo code not working?". If the order is already placed, see "I forgot to add a discount code."
If it still will not go through, message us here, or email [email protected], with the code and what is in your basket, and we will check it at our end.