Rest the door's bottom edge spacers on the sill and tip the closed door into the opening so that the Z-shaped mounting rails cover the side casing's corners. Hold the door with your shoulder as you drill ⅛-inch pilot holes through the middle hole in each rail and into the casing's face; drive a screw into each hole. Then drill pilot holes through the other holes in the rails and screw them to the casing's face.
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⅛-inch diameter drill bit