To redeem a Gift Card or Site Credit, proceed through the checkout process until you are being asked for the payment information. On that page you will see a link that reads, "Have a Coupon or Gift Card?" Click on that, and additional boxes appear for the Gift Card number and security code. Enter the appropriate numbers into the appropriate boxes and hit "Apply."
If the total for the purchase is less than you had on your Gift Card or Site Credit, you will have a remaining balance for the difference. If the total for the purchase is greater than what you have on your Gift Card or Site Credit, you will need to cover the difference with a Credit Card or another Gift Card or Site Credit. Unfortunately, PayPal cannot be combined with Gift Cards or Site Credits.