I have been replacing a lot of internal 1 3/8" doors in my home. Routing out the hinges in the correct location is a little fun, so I created a jig to use the old door as a template.

Place the doors on top of each other and line them up. Clamp the doors together so they can't move. (alignment is critical!) Attach the jig to the old door hinge cut out using the existing hinge screw holes. I use 1 1/4" flat head screws. Then use a router with a standard 5/8" router guide bushing (shown in the pictures above) and a 1/2" router bit. I set the depth of the router to make a 3/64" cut, I find that works very well.