When I have a batch of interior doors and jambs to install, I use a jig that I made out of a cheap hollow-core door. I paid $10 for the 2-ft. 6-in. by 6-ft. 8-in. door, and then removed most of it, leaving about 6 in. of door around its perimeter, as shown in the drawing. The door was pretty flimsy at this point, so I reinforced it between the faces with 1x plywood around its cut edges. Small wood blocks screwed to the corners align the jig with the plane of the wall.
In use, I shim the jamb sides snug to the sides of the door. The opening in the door jig makes it easy to step from one side to the other as I shim the jamb. A perfectly sized and plumb jamb takes less than five minutes to do. When I get ready to hang a door in its jamb, I reduce its width by about 3/8 in. as I bevel the door’s edge.
Steve Prince, Los Alamitos, CA