To redeem your gift card you can use the unique code as a payment method. Go to the final step of the ordering process (choose your payment method) and select the payment method 'Gift Card'. Here you can fill out your code. If you see the button 'Check your code', you know you're in the right place!
If there's still an amount to be paid, you can click on 'Confirm order', and this will take you to a new page where you can pay the rest of the amount.