Since BiblioBucks gift certificate balances can only be used by an account holder, first make sure that you are logged into your registered account with an available BiblioBucks balance. 

Choose your books and add them to the shopping cart. Click "Proceed to Checkout" to continue.

If you are logged into an account with an available Bibliobucks balance, you will see the option to apply your Bibliobucks in the "Payment" section of checkout. Enter the amount you want to apply and click "Apply". You should see that amount of Bibliobucks deducted from your order total.

If your order total is less than your BiblioBucks balance, you will keep the rest in your account.

Continue through checkout and enjoy your books!