So here's a little more on how I built pre-hung doors assemblies using salvaged doors. There may be better ways, this was just the method I came up with. I lay the door on horses opening direction down. After ripping some 1x pine to the jamb thickness I lay them on each side of the door. I use paint sticks as a gap shim between the jamb sides and door, then pipe clamp together to keep stable. Now I can measure for the header piece. I cut the top piece to length and then I router the thickness of the side jamb pieces into each end of the top (half the thickness of the top piece) This leaves a slot on each end for the side pieces to fit into making a nice tight joint. I reassemble the pieces around the door this time using paint sticks for the top gap as well. Now I can mark my cuts for the bottom of the door, which I also had to allow for the thickness that the kitchen floor will be built up. Once these are cut it's time for final assembly. I nail and glue the top joints and shim between the door and jamb assembly again - using pipe clamps as before to hold things together.
I do some quick checks for squareness and then it's time to brace the assembly so it can be moved around for installation. Using the top measurement, I cut three identical length stick (I used scrap from ripping down the jambs). Marking at measured points down each side I then fasten each support to the jamb having the ends flush with the edge of the jamb. This should guarantee that the width stays true from top to bottom of your built jamb. I also add corner bracing at the top. At this point you should have a pretty sturdy assembly and can now mark and mortise your hinges as I showed earlier.
Finally install as you would any pre-hung door. After the unit is installed, just remove the bracing. You may have to do some minor adjusting during the install depending on the condition of the door (it was surely shaved to fit another opening that wasn't square!), but so far I've had pretty good luck with this method and can do one start to finish in a few hours now.