Since there isn't a whole ton of info on this mod I figured I'd contribute a bit (even if it is pretty damn simple.)

Remove the interior trim panel on the wheel well, giving you access to the two bolts that hold the door on
Unscrew those bolts
Remove the gas door from the outside
Button the interior back up
Remove the little rubber pieces in the gas filler area
You may need to shim the filler neck because it is at a different angle than a TJs
I shoved a 3/8s ish thick nut on the top portion and JB welded it in place.
At that point I just fitted the piece then drilled and tapped the holes
The four inners I used 10-32 1" pan head machine screws
The four outers I used 10-32 1/2" flat head machine screws
They match the holes very nicely
Only screw fitment issue was with the outer bottom two, they don't quite align up with the steel, so I just wedged them in (they sit at a bit of an angle, but they look decent.)
If you wanted to you could do some dremel work and fill in with some epoxy putty then drill and tap to get your screws in straight.

Anyways, a couple crappy phone pics:

 

I just watched this mod being done on YouTube. It was done by RYANVISION, he has a bunch of ZJ videos on the tubes. He his kind of long winded though.

 

Cool, thanks for letting me know. I just watched it, and he really goes in depth with cutting and modifying to do this. I guess it makes the trim really flush and nice, but it seems like he hacks it up quite a bit. On mine there is a barely noticeable gap on top, and on the bottom (which you have to crouch down to see) there is a 1/8ish" gap. Which could be greatly improved by modifying it to thread the bottom two screws in. Either way, I didn't have to cut anything, just drill and tap some holes.