Here are some screenshots, I'm not exactly sure what your question is, but maybe they'll help...
Then enter your gift card code and hit "submit".
Just like js250 said, you should then see your total go down as you apply gift cards. (You can use more than one by doing the same procedure again.)
You may have to enter your credit card info still, but it won't be charged if your gift cards cover the total of your order.
Does this help?