in all seriousness there's nothing easy to it. assuming you have all the doner parts you need. The rear hatch of an XJ is Fiberglass until the redesign and not weldable. Even if it was you still have to attach your rear suspension onto something that's not going to fold up like a taco the first time you cross some rail road tracks.
To do a proffesional job on the other hand on a rig you plan to drive and use regularly you need an MJ to cut up, some frame rails need to be grafted to the front 4 door XJ to mount the bed and suspension to, or you need to lengthen the MJ frame (a long bed and wheel base MJ might be usefull here). and graft the 4 door on. you need to stretch the wheel base fill in panels where the wheel well used to be on the XJ graft the back of the MJ cab behind the XJ far enough back that the XJ's rear seats are still useable. which means your cut on the XJ and the cut on the graft have to include more sheet metal (you can't just use the rear window line as a stopping point like some "back half" chop job trail rigs do; the seats go further back than that) Unless you want to put Compact truck king cab jump seats back there.
Lining up those huge grafts will be a problem, and will require a few man days of welding thin sheet metal, all the way around, dressing the welds and then seam sealing them against weld porosity and unless you are a master sheet metal fabricator you will have hundreds of hours of body work ahead of you before paint. And that's to say nothing of stretching your wheel base and suspension work (remember its much heavier than it used to be). It also doesn't address the rear doors which have a wheel arch in them that is now funny looking and out of place.