Inserting measure at start of finished project

This is such a basic thing to want to do but I just can't remember how to!
I want to create some backing tracks for one of my guitar students and plan to do this by lifting off everything but drums and bass from some old projects saved as bun. files; no problem there.
But I'll want to put a couple of count -in measures at the beginning of the project and can't get my head round this process, what's the simplest way of doing this?
C'mon chaps help this old geezer out!
Thanks, Gareth

Thanks Beagle! I was always doing a weird cut and paste trick that had sooo much more room for error. I just never got around to asking this question.