1. Connect in bootloader DFU mode
2. Add to appeared drive file from https://micropython.org/download/SEEED_XIAO_SAMD21/
3. Setup Thony https://thonny.org
4. Test sketches: https://wiki.seeedstudio.com/XIAO-SAMD21-MicroPython/
5. Save with Tony Save as... your python script as boot.py or main.py if no proper loop inside and need to rum continiously

 

6. Micropython guide for XAIO SAMD21 :https://github.com/Seeed-Studio/wiki-documents/blob/docusaurus-version/docs/Sensor/SeeedStudio_XIAO/SeeedStudio_XIAO_SAMD21/XIAO-SAMD21-MicroPython.md 

Some UF2 files for XAIO SAMD21 (absolutely no guarantee) XAIO_SAMD_some_UF2_files.zip