If you have a promotional voucher/coupon which entitles you to a discount off your order, you can enter this from the final order summary page.
Click on the 'Redeem a promotion code' link in the 'Additional Services' section of this page and you will be taken to a page where you should enter the promotion code and click 'Continue'. Any discount that is applicable will then be applied to your order and show as a discount.
Please check the conditions of your voucher if it does not appear to work on your order as some have special conditions.