Set the following settings in Arduino IDE:

Tools → Board → Sanguino

Tools → Processor → ATmega1284 or ATmega1284P (16 MHz) (this should be the second from the bottom)

Tools → Port → Whichever COM port shows up

Tools → Programmer → Arduino as ISP

Select Tools → Burn Bootloader. The lights on the Uno should flash.

If you unplug and replug your USB cable, the printer's LCD screen should be blank, because we've changed the bootloader.