Disconnect the TX/RX wires from the Duet.

After downloading and installing the CH340 Driver from the FYSTEC Github

Add a jumper to "UVCC to 5V". If you skip this step you will not be able to flash the board.

Download the sketch from Github and extract the zip file. Open it in Arduino IDE

Board is "Arduino Pro or Pro Mini"

Processor is "ATmega328P (5V, 16 MHz)"

Upload the sketch to the board

When upload is complete, remove the jumper.