Gift Cards can be entered at checkout as a method of payment.
You will be asked to enter the long 16 digit number and 4 digit pin on the back of your Gift Card. This will then show the available balance on the Gift Card.
You will then have the option to enter the amount on the Gift Card you wish to use against the order.
Any remaining balance after a gift card balance has been applied, can be paid via credit/debit card or PayPal. Unfortunately, you are unable to use Klarna as part-payment when using a gift card payment.
Please keep hold of any gift card used for the purchase, as any future refund will automatically return to the same card. Details can be found here.