I recently moved into an old farm house. Years ago, an addition was added to the front, but apparently was not done well as it has sunk, creating pitched floors and uneven door frames. The doors most affected are the front door and bathroom door. For now, I'm concentrating on the bathroom door.

The frame is not even. The floor is not even. The door barely fits. It grinds against the side and has gaps at the top and bottom. The door has a hole kicked in at the bottom and is just all around nasty. I want to replace it, but I have no idea how to measure a door to fit in such a sketchy frame. Obviously, since the door on there now doesn't fit, I can't just trace it either. Any tips would be greatly appreciated! I have some old doors that I can practice on too.