If you have not yet started creating your product, you can create it via "My credits". When your creation is ready, you'll be able to select the options that corresponds to the credit you have purchased before placing your creation in the basket (eg matt laminate cover for a photo book or Photo Days for a calendar).
When you have placed your product in the basket you'll see that the title of your credit will appear underneath your product with the mention “Redeem credit”. By ticking the box next to the credit title, you should see the discounted price displayed next to “Now”.
If you would like to use your credit for a creation that is already saved on your account, please go to "My Account" - "My Creations" and click "edit" below the product that you'd like to order. Then follow the same process as above, selecting the extra options.