Be warned! The credits to Guitar Hero 2 could very well be the longest credits in existence!
Once you've completed the game the credits will roll and upon them finishing the achievement will unlock, however it is possible to unlock them earlier.
To unlock the achievement before completing the game you need to select "Options" from the main menu and in there you can choose to watch the credits, doing this will still unlock the achievement. Be warned though, whichever method you decide to use you'll have to watch the entire credit roll for the achievement to unlock and the credits last for approximately 25 minutes.
I've linked in a video of the credits. It's a shortened version but it'll show you when then achievement will unlock.
*Credit to henrymetal15 for the video