This is an old revision of the document!
Flash guide: https://www.youtube.com/watch?v=fN_QKOWvG1s
To flash a new firmware to your Sonoff, you have to boot your Sonoff in flashing mode. Follow this 4 step process:
1) Connect your 3.3V FTDI programmer to your computer
2) Hold down the Sonoff button
3) Toggle the switch to apply power to the Sonoff circuit
4) Then, you can release the Sonoff button
Now, your Sonoff should be in flashing mode and you can upload a new firmware.