M5Stack | ESP12
 ------------+------------
    16 (Rx2) | D1 (TX0)
    17 (Tx2) | D3 (RX0)
          12 | D0 (FLASH)
          13 | RESET
        3.3v | VCC
        3.3v | D2
        3.3v | CH_PD (EN)
         GND | GND
         GND | D15

• Compile & Upload this sketch on the M5Stack
• Change board to ESP8266
• Set Baudrate to "115200"
• Set Flash Mode to "DOUT"
• Set Reset Method to "no dtr (aka ck)"
• Open your ESP8266 sketch
• Hit the Button B on the M5 so the ESP8266 waits for flashing
• Hit the Upload button

Also acts as a serial bridge (115200) after flash timeout.