When you get to the last stage in the order flow, you'll see your order summary on the right side of the screen. Right below the products you've selected, you'll see "Gift Card or Discount Code."
Enter the code there. Then make sure you push apply!
Once you do that, it will reflect the adjusted price and you can check out as normal from there.
If you have any other questions, just give us a shout at firstname.lastname@example.org.