^^ I havent made any tut on these, but below are some pointes for you to get started ...
1. Everything is integrated and controlled by Home Automation OS called Home Assistant running on Rpi. (just google Home Assistant on Rpi)
2. HA supports truck loads of components, so if you have already existing devices lilke Philips hue or wemo or in fact an automation hub as well like fibaro or vera you still can use HA to include those as well.
3. Once HA is installed, you need to configure it by configuration.yaml file (all tut available on website) that yaml file will tell HA what devices to look and connect etc etc.
4. Coning to sonoff : sonoff initself are independent devices, with their own app that also works with alexa, so all you need is to connect it with the device download app register it and u r good to go. BUT, to make it work with home assistant, what is what makes it worth, you need to update it with CFW. Tasmota is the best out there. Now, this procedure requires a bit patience and reading. But once the CFW is uploaded, all you need is to update your YAML file and the device will start showing on
HA dashboard.
5.Last part is Alexa part which is the easiest. Once HA is working and all devices are added and listed, all you need to do is to ask Alexa to discover them all and she will. Then its just a matter of saying Alexa switch off device name that you gave in HA.
thats all .... try and best of luck.