Pre-hung doors make installation much easier, but what if you want to just replace the door blank without removing the casing and jambs? How do you get the door to line up properly? At least for the hinges, you can align them with a butt marker.
Stanley makes butt markers for 3″, 3-1/2″, and 4″ hinges. They make their butt markers from steel and finish them with a black baked-on enamel.
To use one, align the butt marker on the door or jamb with the existing hinge on the door side of the jamb, and strike it with a hammer a few times to leave an outline for creating the hinge mortise.
You’ll pay about $6 to $9 for the correct butt marker, but if used properly, it’ll make the process go a lot smoother.