1. Download and install the latest version of Arduino: https://www.arduino.cc/en/software/
2. Download our 300x Firmware: https://github.com/makermadecnc/Marlin2.0/archive/refs/heads/master.zip
3. Unzip "Marlin2.0-master.zip"
4. Click on the folder labeled "Marlin" then double click on "Marlin.ino". It should look similar to this:
5. Click on Tools > Board > Arduino AVR Boards > Arduino Mega or Mega 2560
6. Then Click on Tools > Processor > ATmega2560
7. Then click on Tools > Programmer > AVRISP mkII
8. To finish preparing the firmware, click on the white check mark in the top left to verify.
It will take several minutes to compile.
9. When the firmware is ready, it will say "done"! If an error occurs, and is says that it cannot compile, check that the correct board, processor, and programmer are selected.
10. IMPORTANT: Make sure the printer is power off. If the printer is powered on, the Arduino will not be able to communicate with the board.
11. Plug the USB-B that came with your printer into your computer and the front of the 300x.
12. Select the port the the USB is plugged into on your computer. Choose the one labeled "usbserial" with numbers next to it.
13. Click "Upload" and the firmware will upload to your printer! If the upload fails, ensure that the correct "usbserial" port is selected and that no other programs are open that communicate through USB Serial (such as Cura or Makerverse). If you have to close other programs, you may have to close, reopen, and recompile Arduino.
14. Arduino will notify you when the upload is complete. Unplug the USB cord and turn on your printer. The firmware is now flashed!