You have a giftcard to use? Lucky you!
At the back of the card, underneath the barcode, you'll see a long number. In the shopping cart, underneath your products, click on "Do you have a discount code or gift card?" Type in the number into the field and click enter. Your gift card will be calculated into the order amount automatically.
If you're not using the full amount of money on the gift card, make a note to yourself how much is still left after the purchase! You can type in the number again when you're ordering the next time and make use of the remaining amount!