Measure the old door and remember which kind of swing the door has now: Does it swing to the left or the right?
Take the measurements to your local home improvement center or lumberyard and pick out a door to suit your needs. Someone who works there will be able to help you get the right size and the right swing - that is, the direction the door opens in, and whether you push or pull it to open.
Remove the old door by taking it off its hinges. Next, carefully remove the trim work from around the door and set it aside. This will save a little money and the problem of trying to match finish products on new wood.
Remove the old door frame from the opening.
Set the new door unit in the opening once clear (leave the new door on the hinges, as this will help keep the unit square). Use a carpenter's level to make sure the unit is plumb and level.
Use wooden shims to hold the door in place. Check to see that the door opens and closes freely before nailing the frame in place.
Replace all trim removed earlier. Finish the door and frame to your satisfaction.