First you need a controller. It can be a PC USB controller, or a Playstation 2 or other console controller as long as you have the proper adapter. I got my adapter cheap off of eBay, and it works great.

Now you'll need a free program called JoyToKey, which allows you to map keyboard function to any game pad. Just put JoyToKey into Google and choose the first link that says 'JoyToKey English Version' and download the program.

Now, with your guitar controller hooked up, I went to Start-Control Panel-Game Controllers and double clicked on my controller under Installed Game Controllers. Pressing the buttons revealed that the green, red, yellow, and blue frets were respectively buttons 6, 2, 1, and 3. Go figure.

Anyway, with that figured out, I opened up JoyToKey. There are different ways to control the car in Audiosurf, so figure out which one you want to map to the keys. I wanted mine to match up to the a, s, d, f setup, where a is the far left lane, s is left, d is right, f is far right, and it defaults to the center lane. If you want you could map the left and right keys instead to switch lanes one at a time. Anyway, click Create at the bottom left of JoyToKey, and name the new setting Audiosurf. Remembering which frets corresponded to which button numbers from before, click on the button number and set the key that you want to map to it. Then leave JoyToKey running, start up Audiosurf, and you're ready!

Now, some of the cars have special abilities that require other keys. Feel free to map these to any of the other buttons: you still have the strum bar up and down as well as the start and select buttons. You can easily figure these out in the control panel screen the same way as the frets.

Also, any controller will work. Another good setup is using a standard gamepad with four shoulder buttons. You can use the four shoulder buttons in the same way I used the four frets, or use the directional pad instead. JoyToKey will make any controller work, so use anything!