This story shows how to fit a new door to an old opening.
If an existing door is damaged or worn, you can put a new door into the doorway. Often the easiest way is to remove the existing jambs and casing and install a prehung door. However, if the doorway is square or if the existing trim is difficult to replace, you can keep the doorway and replace the door.
Unless your house is very old, you'll have little problem finding doors to fit. The most common door height is 80 inches, although most are available 78 inches tall as well. Common interior door widths are 24, 28, 30, 32, and 36 inches. Height and width are often stated in feet and inches: 2-6 for 30 inches or 3-0 for 36 inches, for instance.
Check that the doorway is square. If not, scribe and plane the door. If the doorway is well out of square, remove the casing, cut through the nails holding the jambs, and reattach the jambs so they are square.
About 3 to 4 hours
Tape measure, block plane, square, router, chisels, drill/driver, 21D8-inch hole saw, 1-inch spade bit, utility knife
Measuring and laying out, planing, cutting mortises, drilling
Remove the old door, check the opening for square, and acquire new hardware if necessary.
New door, new hardware (if needed), shims, matchbooks
Check the door fit. Ideally there should be a gap of about 1/8 inch at the top and along each side, and about 3/8 inch at the bottom. Use cardboard spacers or folded matchbooks (four thicknesses equals about 1/16 inch) along with shims underneath to maintain the spacing. Mark the mortise locations and cut the mortises.
Hang the door. New locksets come with a template to help you locate where to drill holes. If you are reusing a lockset, extend a line across the face of the door with your square, making sure it is centered on the strike plate. Measure the lockset to determine the distance the hole should be from the edge of the door. Drill a 2-1/8-inch hole through the door with a hole saw.
Insert the bolt into the hole in the door's edge. Align the bolt plate with the edges of the door and trace around it with your utility knife. Remove the bolt and remove the wood inside the outline to create a mortise for the plate. When you're finished, the plate should be slightly below the surface.