When you reach the checkout screen, you can type or paste in the discount code into the field next to 'Add a discount code'. You need to enter the code exactly as provided, without any spaces or additional characters.
Once you've entered the code, click on 'Apply' and it will add the code to your order, if valid. When the code has successfully been applied to your order, you'll see the green box below the discount field showing your applied discount code, your saving and the total of your order will change to reflect the code.
Please note, you must enter the discount code when you reach the checkout screen as it can't be applied later.
If you are unsure why your discount code has not been accepted, please see Why has my discount code not been accepted?