Within your bag and before you select a delivery or collection option, click on 'Promo Code' and the below box will appear.
Enter your promo code in the 'promo code' box then click the apply button. If your code is valid, you'll see your discount in your bag on the right hand of the screen.
If an error appears, check that you have typed the code correctly and any expiry date. Please note, some codes are not valid on sale items and this will always be specified in any terms and conditions issued with the code.