If you have a gift card or a voucher, that you want to use towards the payment of an order. Then do as following:
On the fourth step of the checkout, you should see a checkout summary. On this page, you will see a headline: "I have a voucher/gift card" - here you can see a small field, saying "Enter voucher/gift card code".
By clicking on this, you will get the following picture:
Here you enter the code/gift card you have and click on "redeem gift card". That's it!