First of all determine if the entire jamb and door needs to be replaced or just the actual slab. If only the slab needs to be replaced determine the size of the door. Measuring just the slab of the door from side to side will give you an accurate measurement. Most interior doors are all the same height, eighty inches. Proceed as follows:
Gently tap the hinge pins from the existing door with a screwdriver and a hammer. Slide the pins out of the hinges while someone supports the door. The old door should come off without too much trouble.
Carry the old door to your work area, and remove all of its hardware with a screwdriver or a power driver. Place the old door on top of the new door you are going to replace it with. Since the old door fit into the doorframe, you can use it as a template to cut the hinge mortises and bore the lock set on the new door.
Make sure that all four edges of the two doors are perfectly flush and then clamp them tightly together.
Begin by transferring the hinge locations. Use a combination square and lay it on the edge. Instead of using a pencil, use a utility knife to mark the location of the new hinge. This will give you a much sharper line and it will give you a place to set the chisel.
After marking the sides if the hinge mortise, use the combination square again to scribe the line across the bottom.
You will need a hole saw to cut out the large hole for the latch cylinder. Use the old door as a template so you will know exactly where the hole needs to be made. The thickness of the door will help keep the drill vertical. Push firmly down as you saw and stop drilling as soon as you see the pilot drill comes through the other side of the door. It is helpful if you can have someone else watch the underside while you drill.
Flip the doors over and use the small pilot holes as a guide and continue drilling from the other side. This will create a clean edge on both sides of the hole, which is well worth the extra effort.
Note: It is important to hold the door firmly in a vertical position while working on the hinges and the latch hole. Use a spade bit to bore a latch hole. This hole must be perfectly vertical. It is helpful to use the combination square as a guide.
Screw the latch bolt into place temporarily and scribe around the outside of the plate. Then it is time to do some chiseling.
First cut the edge of the mortise. The knife mark made earlier provides a shallow V into which you can set the chisel blade. This will give you much better accuracy than a pencil line would. A chisel has two sides, the bevel and the flat side. Remove material by placing the bevel side down. As you get closer to the proper depth, turn the chisel over with the bevel side up. With the flat part of the chisel against the flat surface you are working on, you can pair a little bit at a time to create a flat surface for the hinge.
When the latch bolt mortise is complete, turn the door over to make the hinge mortises as described above.
With accurate marking and careful chiseling, the hinges should fit perfectly.
After attaching the doorknob set to the new door, it is time to hang the new door. With a few taps on the pins the door-hanging project is complete.