Measure the existing door carefully, then go to your preferred supplier. Get the door that is closest to your actual measurements (round up - don't get a smaller one!).
Take off the target door by removing the screws from the part of the hinge that is in the frame. Remove the handle and any other protruding part if applicable. Clamp the door to the new door and check for overall size match. If it needs trimming, take an equal amount off the top and bottom to adjust the height and do the same with the width.
Once doors are equal, mark up for the hinges by removing them from the existing door and clamping the doors together again. If the top isn't obvious, mark it with a bit of tape. Scribe across for the hinge positions, mark up and remove the mortise as required - make sure you mortice on the correct side of the door!
Now clamp the doors together side by side and mark up for the latch/lock position. Mortise for the latch/lock as required, then drill for the handle/keyhole. To mark this hole, offer up the latch/lock on the outside of the door, parellel with its mortise and with the face plate aligned with the door edge. Mark through the centre of the handle/key hole with a bradawl or similar.
Providing that you hold the tool perpendicular to the door and eyeball the hole centre, you won't go far wrong. There needs to be sufficient hole for a square spindle to turn in it, but not too large to weaken the lock mortise or exceed the handle boss diameter.
For pics associated with this task, go to
http://www.diyfixit.co.uk/ click on "Carpentry" (fifth link down) then "Door hanging" - that should help.
Ray.
