You need to make a purchase first, as long as you believe that you will pass a background check. If you buy online, the firearm is shipped to your local FFL, and you will complete the background check then. If you buy locally, you will complete the background check after purchasing the firearm you want. The serial number and type of firearm (handgun, rifle, etc) is listed on the background form (ATF Form 4473).

You make a purchase, then choose a local FFL where the firearm will be shipped. When they receive it, they contact you and run the background check. If you make a purchase through Bud's, they have a list of recommended FFLs in their checkout process.