The first step is to make the door fit the opening or vice versa. The door should be about 2 1/4 inches narrower than the rough opening and 1 inch shorter than the distance from the header to the finished floor. If you have to cut a door a significant amount in width, trim from both edges to keep the stiles symmetrical. With hollow-core doors, try not to cut beyond the solid edges. If you have to cut into the hollow area, save the solid edges and glue them back into the door to reinforce it. To do this project efficiently, devote one and three hours of time and be ready to measure accurately, use power tools, and drive screws. Before you begin, make sure the rough door opening is square and the used door is square and flat. Project tip: 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.
