Many doors will not need a hinge router, while other doors only have a slight marking for the hinges, and others have nothing at all. Many professional door hangers won't even use a router for door hinges but instead trust the handy wood chisel, which is always a part of their tool bags. For the novice to moderate user, a router makes installing the hinges much easier and more accurate, ultimately resulting in a better hung door.
Step 1 - Plumb the Door Jamb
Take a carpenter's tri-square or measuring square, and check that the door jamb itself is on an accurate 90-degree angle. If the angle is less than 90 degrees on the hinge side, you may be able to pull in the lower portion of the jamb using 1 1/2-inch wood screws or longer. If the angle is greater than 90 degrees, use cedar wood shims to build out the door jamb and square it up, a process known as "plumbing" a door jamb.
Step 2 - Measure the Top Hinge
Measure from the top inside corner to the top of where the hinge will go on the jamb. On the door, pull "down" from the top hinge side of the door the distance you just measured, and add 1/16-inch, to allow for some minor swing shift for the door. Many professionals will tell you to begin by measuring up from the floor, but you cannot always consider the floor a level surface. The door will always have to fit the door frame.
Step 3 - Measure and Mark the Remaining Hinges
Repeat the top-down measurement for the bottom hinge and for the middle one as well, if the door uses a triple hinge. For each hinge location, make a small mark at either side of the door's thickness. Lay your measuring square across the thickness of the door so the straight line matches a set of marks and draw a line across.
Step 4 - Imprint the Hinge
Position one of the hinges on the door in the manner it will be mounted. Line up the top of the hinge with one of your two or three hinge marks. You can now mark the hinge with a pencil, or tap your hammer lightly around the edges of the hinge. The impression of the hinge you leave in the door will be perfect for routing. Repeat this process for each hinge. To save time later, run a hinge screw into each location where they will go later.
Step 5 - Router the Door
Set the routing bit to cut a depth of 1/8 inch. Place the router on the edge of the door and carefully route out each of the hinge depressions you made. If you have trouble getting the right curve or turn for corners, don't worry. You can easily touch these up with a utility knife. Once the holes are routed, test fit a hinge. Use a utility knife, wood chisel, or further routing to adjust the fit.