git clone https://github.com/adafruit/circuitpython/
cd circuitpython/
git submodule update --init --recursive
make -C mpy-cross
cd circuitpython/ports/atmel-samd/

The default board is debug. You can build a different one using:

make -j8 BOARD=stackrduino_m0_pro

Further instruction: https://learn.adafruit.com/building-circuitpython/build-circuitpython

• Attach StackRduino M0 PRO to your computer via USB.
• Double Tap 'reset' button to put board into UF2 Bootloader mode
• Copy firmware.uf2 from the build folder to the USB Drive called "StackRduino".
• Wait for CircuitPython to be installed (about 5 seconds).
• Done.