Measure the width and height of the door with a measuring tape.
Measure the width of the door molding with a measuring tape. Add the width of the molding to the width and height measurements to determine the length of the molding sections.
Cut two sections of molding to the height of the door, plus the width of the molding, with a circular saw. Cut an additional section of molding to the width of the door, plus twice the width of the molding.
Miter the top end of each height section at a 45-degree angle with a miter saw. Miter both ends of the width section at a 45-degree angle.
Place one of the height sections at each side of the door and secure the section to the edge of the door frame by driving finishing nails through the molding at 4-inch intervals. Place the width section at the top of the door frame and secure it with finishing nails and a hammer at 4-inch intervals.