First, download the songs.
1. If the files you have downloaded have extension .zip or .rar, download WinRAR and install it. Afterwards reboot your computer and right-click on the downloaded file. Pick the option Extract here. If you already have it or other archiving program, you do not need to install WinRAR.
2. Check if there are following files in the archive:
If song.ogg is not present, simply copy guitar.ogg and rename it song.ogg afterwards.
If song.ini is not present, make one yourself in this way:
[song] name = Walk This Way artist = Aerosmith cassettecolor = #F1D6C3
Replace the name and artist fields with the name and artist of the song. Replace the cassettecolor field with one of the hex colors from this site.
3. Place the files in the directory
NOTE: "FoF_Folder" will be named whatever it was downloaded as, generally "FoF" or "fretsonfire"
Replace NameOfYourSong with the name of song you have downloaded.
Video on how to add songs.