In this Instructable we will use a handy program called The Ghost to inject custom songs into Guitar Hero 3 (or Areosmith) on Wii. These games may then be burned to a CD or put on a flash drive to be played on a soft or hard modded Wii. The instructions for modding your Wii and running/copying Wii ISO files can be found here (soft-modding Wii) and here (making/running backups). This instructable works for Windows XP. Also, read the disclaimer.
NOTE: A.C.E. has been kind enough to make an instructable on this topic already. This instructable is meant to address problems and updates to the software. Also I did not write The Ghost, see The Ghost's official page here.
DISCLAIMER: AS WITH ALL WII HOMEBREW, BE CAREFUL WITH WHAT YOU DO TO YOUR WII. I AM NOT IN ANY WAY, SHAPE OR FORM RESPONSIBLE FOR YOUR WII. ALSO, COPYING WII GAMES IS ILLEGAL. NUFF SAID. PERIOD.
Well, now that that's out of the way, let's get started!
Make sure to look at the pictures and the notes in the little yellow boxes. Although I make sure to explain everything in the text, look at the pictures for additional details. Also, if you click the little "i" in the upper right hand corner and look for the "Original File" link, you should be able to see a larger, more detailed image for most screenshots. Other than that, remember that software is still only a piece of software and not-fool proof. It may not work. Try to find a solution either in this instructable or by searching Google.