Measure the door frame at the top of the staircase with a measuring tape. Measure the height and width of the door to ensure that it fits in the door frame. The door should be about 1 inch shorter than the height of the door frame to provide floor clearance.
Cut a section of 1-by-2-inch lumber equal to the width of the door frame with a circular saw. Lay the lumber section on the floor in the door frame and place the door on top of the lumber section.
Mark the position of the latch on the door frame. Mark the positions of the hinges on the opposite side of the door frame.
Cut a 1-inch hole for the latch in the side of the door frame with a router. Place the latch plate over the hole. Insert the included latch plate screws through the holes and tighten the screws with an electric screwdriver.
Position the hinges on the opposite edge of the door. Attach the hinges to the door with the included screws. Attach the hinges to the door frame with the included screws.
Remove the lumber section from beneath the door. Open and close the door to ensure that it functions properly.
Insert the knob and latch assembly through the pre-cut hole on the latch side of the door. Insert and tighten the assembly screws to tighten the assembly.