For the coupon to be applied, the qualifying product(s) must be added to your cart first.
There are two ways to add a coupon code to your order:
Apply coupon code on 'My Basket'
Once you have added the product(s) to your cart, click on 'My basket' > 'View and Edit Basket'. On the grey box on the right, there is a 'Apply Discount Code' section where you can add the code and hit the 'APPLY DISCOUNT' button. You can then click 'PROCEED TO CHECKOUT' from the button below.
Apply coupon code on Checkout
Alternatively, click on 'My basket' > 'PROCEED TO CHECKOUT'. On the next screen, fill in your Shipping Details, select a Shipping Method and click 'NEXT'. Before selecting a Payment Method, click on 'Apply Discount Code'. A field will appear where you can add the code and hit the 'Apply Discount' button. You can then select your preferred Payment Method and check out.