I was asked, by a singer friend, if I could create two backup song tracks for her.  I've mixed down a couple of tracks for her off of a CD project I did for her, just eliminated the vocal track from the mix, but never tried tracking specifically for backup.
 
I downloaded the songs she wanted and changed them to the keys she wanted using my Karaoke program.  I then started making the individual tracks using the song as reference, so everything would be in sync for vocals.  I did the Drum track using drum pads on my Alesis SR-16 drum machine.  I tracked bass, Pedal steel guitar, rhythm and lead guitar.  I didn't particularly like the mixed "backup" tracks but the singer was happy with them (which I guess what counts).
 

So you basically made a Karaoke track. 

Can't you BUY karaoke tracks?  Seems like an awful lot of work to recreate something that's already there...

There wasn't any commercial Karaoke tracks available for these two (gospel) songs. 
I didn't actually make a "Karaoke" track as I don't have the software to create the lyric track (or at least what I consider a "Karaoke" track).  Just a "backup" track.
 
I wouldn't do this on a commercial basis.  Just did the two for a friend.  But, it was a challenge to see if I could do it.