I'm really not into giving "step by step" instructions on how to do this, I'm more of a "here's how I got here and if you have a better way, please share" type of person.
So I started with a 97 Isuzu Rodeo Tire carrier, 8' of 1 1/4" square tubing and some 10 mm nuts(which I ditched later) and my trusty wire welder.
I took a picture(right) while I was at the junkyard for reference... Basically the body was thinner than my Jeep's sheet metal and under-structure was where the beef was, which in comparison the 1 1/4" square tubing was overkill. For a total of 5 seconds I was thinking of cutting out the Rodeo's under support, but then I remembered the Junkyard's no power tool policy, plus where would the fun be in that. Getting the tire carrier was by far the easiest part of this job.
So now I'm back in the garage and I start this job by prepping the body; I ground off the paint where I'll be welding, deciding the general location of where the carrier was going and cut holes in the body where the hinges go. On the hinge-to-body area of the carrier I started grinding the hinges flat(Rodeo's body is round/convex the Jeep is flat) to fit the hinge tight against the body. I then started making the under-body support; fitting the two pieces that fit directly against the hinges first, then dry fitting the carrier on the body, I cut and tacked the rest of the support together. Here's where I made my first mistake in not including the gasket in the fitting, but I just ended making a spacer for the under-body support to fix it.
After taking the carrier off the body I finished the welds, cleaned up the under-body supports and checked the fit(left) against the hinges. I put the carrier back on the body and tacked the under-body support in place.
Sometime this week I'll post the rest which is just: mounting the latch on the tale gate, adjust, tweak, adjust, install new tail lights, and prime any bare metal.