From here, you'll be brought to a checkout screen. Choose the option you'd like to apply the promotion to (note, most promotions only work when applied once!) and then select the "Have a promocode?" link indicated in the image below. Enter your promotional code in the text box that appears and hit the "check" button next to it to confirm that the promotion is still active. After this, you're all set! Just hit the "Billing Info" button below and finish the checkout process as you typically would.