YouTube - How to do an overlay in GIMP (Warning: contains excruciating music) I don't use gimp but I am guessing that it is a very similar method to photoshop, if that doesn't help then you should contact some of the top notch designers on here that use both programs. As I said, other than providing that video I cannot help. I hope this helps tough, good luck!
I watched this, but Its an older version that he has. I use gimp 2.6 Edit: I tryed a tutorial, I will post my results in a second.