How to do it
Measure and Cut
Mark Mortise Locations
If a used door must be trimmed to size, cut or plane the edge to remove the old mortises. This prevents having to find hinges to match the old mortises. Most doors require three hinges (hollow-core doors need only two). If neither the door nor the jamb has mortises, center the middle hinge and place the other hinges about 6 inches from the top and bottom of the door.
Cut the mortises in the door first. You can cut them with a router or a chisel. Clamp some 2x4 scraps on both sides of the door to keep the router from tipping. To make the job easier, buy a hinge mortise template for routing. Routed mortises have rounded corners; square them with a chisel if your hinges have square corners. Transfer the locations to the jamb.