Instructions
Upload your own .ogg files to the game by moving the .ogg file to the directory "c:\program files\frets on fire\data\songs\name_of_song," where "name_of_song" is the name of the song. Open the game and select "Song Editor," then select the song you uploaded to edit the song's notation.
Insert your Guitar Hero or Guitar Hero 2 DVD and open Frets on Fire. Go to "Song Editor," then "Import Songs" and "Import from Disc." This takes a few hours, depending on your computer's speed.
Download songs from a fan site such as "fretsonfiresongs.com" and extract the .rar file to "c:\program files\frets on fire\data\songs." Usually songs that have been converted already have correct formatting, including individual directories for each song.