Dell 5210n Mono Laser Printer User Manual page 106

NOTE: Do not remove a tray when a job is printing or when the operator panel indicator light is blinking. Doing so may cause a jam.
2. Squeeze the side guide tab, and slide the guide to the outer edge of the tray.
3. Squeeze the rear guide tab, and slide the guide to the correct position for the size media you are loading.
4. Flex the sheets back and forth to loosen them, then fan them. Do not fold or crease the print media. Straighten the edges on a level surface.
5. Place the print media in the tray:
Print side face-down for simplex (single-sided) printing Print side face-up for duplex printing