This story shows how to hang an old door in a new jamb.
If you want to keep an old door but the old jambs are damaged or you want to install an old door in new construction, purchase a door frame, sometimes called a jamb kit, at a lumberyard or home center. The kit consists of three pieces of lumber 9/16 inch thick and 4-3/8 inches wide. The two side jambs are milled across the top to receive the third piece, the head jamb.
The first step is to make the door fit the opening or vice versa. The door should be about 2-1/4 inches narrower than the rough opening and 1 inch shorter than the distance from the header to the finished floor. If you have to cut a door a significant amount in width, trim from both edges to keep the stiles symmetrical. With hollow-core doors, try not to cut beyond the solid edges. If you have to cut into the hollow area, save the solid edges and glue them back into the door to reinforce it.
About 1 to 3 hours
Table saw, circular saw, router, tape measure, layout square, chisels, drill/driver, utility knife
Accurate measuring and laying out, cutting with power saws, routing hinge mortises, trimming with a chisel, driving screws
Check that the rough door opening is square and the used door is square and flat.
Jamb kit, 2-1/2-inch screws, doorstop molding
Hook the tape measure into the side jamb rabbets or dadoes and measure down a distance equal to the length of the door. Add 3/4 inch clearance for an interior door or 1/4 inch for an exterior door, plus the thickness of any flooring that will be added later. Cut the jambs to this length.