Use a door hinge mortising jig by marking the position for the hinge, adjusting the jig to the correct size, using screws to hold it in place and cutting the mortise using a router with a bearing-guided bit. Use the same jig for cutting the mortise on the door and door frame. Wear safety goggles when cutting the mortise.
When working on the door, support it with the hinge side facing up using two sawhorses. Place the top edge of the top hinge and bottom edge of the bottom hinge at the same distance from the edges of the door. For doors that require three hinges, center the third hinge between the upper and lower ones. Use a tape measure to determine the location of the hinges, and make a mark for the jig. Align the jig with the mark and the edge of the door.
Attach the jig to the door using short screws through the holes the manufacturer provides. Adjust the router cutting depth to match the thickness of the hinges. Move the router in a clockwise motion around the jig to cut the opening. Remove the screw, and move the jig into position to cut the next mortise.
On the door frame, allow an additional 1/2-inch clearance from the top of the frame and 3/4-inch clearance from the bottom of the frame. Most jigs have rounded corners. If using hinges with square hinges, use a wood chisel to cut the mortise corners to match the hinge.