Before trying to trim the door open it and examine the hinges.
Plane hinge side of door.
Turn the door so.
On many doors the grain runs from top to bottom instead of side to side.
That may be difficult on older painted doors.
Use a pencil to mark this area on the side of the door not on the edge.
Test each screw by tightening it with a screwdriver.
Step 2 use your ruler to measure 1 8 inch from the jamb to the start and end points on the door.
But frequently the solution is on the hinge side.
Hold a belt sander plane or power plane at about a 45 degree angle to the edge of the door and remove material to the witness line.
Running the plane in this direction will create smooth cuts instead of chunky bumpy cuts.
Plane the hinge or lock sides of the door in the same direction as the wood grain applying light pressure to remove very small amounts of wood at a time.
The radius of the mortise is determined.
Immobilize it with a doorstop and plane it in place see step 3.
Run a piece of thin cardboard or folded paper in between the door and the jamb to identify the start and end point for the area that needs to be planed.
The hinge master mortising jigs have been designed to route most of the various hinge sizes of todays door industry.
But for maximum control and to plane longer portions take the door off its hinges with a screwdriver or drill.