To Flash, use the esptool from espressif.

(FemtoBeacon: Press and hold boot button)

Plug in board via USB-to-UART adapter

  # Replace YOUR_PORT_HERE with the actual port on your machine. 
  # Can be a COM port on Windows, or /dev/tty.usbserial-**** on MacOS, or /dev/ttyUSB0 on Ubuntu, etc...
  esptool.py -p YOUR_PORT_HERE erase_flash
  

(FemtoBeacon: Unplug. hold boot button, plug back in)

  # Replace YOUR_PORT_HERE with the port, and ESP32_BINARY_FILE.bin with the ESP32 binary for this device
  esptool.py -p YOUR_PORT_HERE write_flash -z 0x1000 ESP32_BINARY_FILE.bin