Stand the door on edge, with the hinge side facing upward. Measure 5 1/2 inches down from the door's top and make a mark. Measure 9 1/2 inches up from the door's bottom and make a mark. If you are installing three hinges, measure the distance between these two marks and divide in half, then make a mark at that point.
Set a hinge template onto the door's edge. The notch on the back of the template butts up to the door's exterior side. Line up the bottom of the template with one of the pencil marks.
Secure the template in place with sheet metal screws. Open up two hinges. Lay one hinge across the template's bottom and the other over the template's top. Keep the center of the template open so you can insert the router bit.
Set the router on top of the two hinges. Lower the bit until it just touches the door's edge. Lock the bit to this height and remove the hinges from the template.
Place the router flat on the template. Turn the router on and gently move it around the template's entire opening. The router miters away wood the exact depth and shape as the hinge being installed.
Unscrew the template from the door's edge. Set the hinge into the mitered space. Clean up the edges with a wood chisel, if necessary. Place the sharp tip of the chisel against the wood you are removing and tap the chisel's handle lightly with a hammer.
Place the template's bottom edge against the next hinge mark and repeat the entire process.