You need to enter your discount / promo code on the cart page. Under your order details, you’ll see “Coupon” – simply click on this and enter your code in the box.
Once you’ve entered the code, click on ‘Apply Coupon’ and it will apply to your order. You’ll be able to check this underneath the items as the discount value will have been removed and the price amended. You must enter the discount/promo code when you are checking out as it can’t be applied later.