Dell 3100cn Color Laser Printer - Manual (Page 18)

5. Attach the straps to the left and right side of the printer, and then close the optional duplex module.
6. Tighten the screws on the bottom left and right while firmly holding the optional duplex module in a closed position.
7. Open the optional duplex module. Remove the connector cover, and then attach the connector on the bottom left of the optional duplex module to the
printer. Keep the original cover somewhere safe.
8. Close the optional duplex module.