Important that you plug in the USBASP converter that takes you from 10 to 6 pins. Place the header pins into the microcade, do not solder them.
Click Tools > Board > Boards Manager…
Type “Arduboy Homemade Package” and select the one by Mr.Blinky. Install it.
Select the right COM port, this is how the USB is connected. The best way to check this is to plug the USBASP in, look at what port number appears, then disconnect and see what number disappears. This will be the port.
Select USBASP as programmer
Make sure the motherboard is connected and press “Burn bootloader”. Wait a moment and it should be flashed.