That may be difficult on older painted doors.
Plane hinge side of door.
Turn the door so.
On many doors the grain runs from top to bottom instead of side to 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.
The radius of the mortise is determined.
Test each screw by tightening it with a screwdriver.
Before trying to trim the door open it and examine the hinges.
People usually try to plane off the edge.
Use a pencil to mark this area on the side of the door not on the edge.
Step 2 use your ruler to measure 1 8 inch from the jamb to the start and end points on the door.
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.
But for maximum control and to plane longer portions take the door off its hinges with a screwdriver or drill.
Immobilize it with a doorstop and plane it in place see step 3.
A hinge can often work loose binding the door against the frame.
Complete one pass per stroke with a hand or power planer with.
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.