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)
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)