Last one

http://benlo.com/esp8266/esp8266QuickStart.html

And then there is this:

https://community.smartthings.com/t/release-sonoff-5-wifi-switch-device-handler/45957

Would get you going pretty quickly (the FTDI explanation is a little more clear that the earlier link with the picture)

I haven't yet made a MySensors node with an ESP, but it's doable

http://forum.mysensors.org/topic/3583/mysensors-on-esp8266-esp01/4

This article shows you how to connect an FTDI to the sonoff

http://tech.scargill.net/itead-slampher-and-sonoff/

From there, I think this tutorial will get you going

https://learn.adafruit.com/adafruit-huzzah-esp8266-breakout/using-nodemcu-lua

leading well into this project on Github, which, I think, is what you were referring to for MQTT on the Sonoff (all of this, of course, removes the original cloud functionality)

https://github.com/elric91/nodemcu_sonoff