First, he created a file with a JPEG of the vintage Apple logo and then sent it to Gelaskins' upload tool. From there, the process is simple. Using the online tool, drag the uploaded image to the iPhone 4 template and set it just so. You can even create matching wallpaper. Finally, make your purchase and let the Gelaskins worker bees do the rest.
Yes, it's just a stick-on skin, but the vintage logo ads a bit of cool and, unlike Engadget's solution, all of the original parts are intact.