Dell 3100cn Color Laser Printer - Manual (Page 5)

6. Attach the trays together by installing the standard tray you just removed on top and inserting the two screws provided. Tighten the screws with a coin
until they are secure.
7. Place the printer on top of the tray modules.
8. Lock the trays into place by removing the top tray from the tray module and rotating the locking mechanism as shown in the graphic.