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Mark the location of the hinge on the door.
Use a fluting bit on the trim router with the cutting depth set to the thickness of the hinge.
Place a template over the door. Build the template with legs that fit over the door. Cut a hole in the top piece that works with the base of the router. (The template limits the movement of the base so it cuts only within the confines of the mortise.) Make an alignment mark on the template to indicate where it should be aligned with marks on the door.