I wanna summarize you my experience with sonoff - Tasmota flashed modules.
There are basically three steps I pass throught before be able to integrate these modules as I image to do and I wanna explain you in a brief:
Step 1) Flash Tasmota firmware on the sonoff and run as stand alone switch on a stand alone cable.
(I assume you are already at these level. Otherwise I suggest to stop read these post)
Step 2) To Integrate the sonoff module for a simple single point light switch
I procede as shown.
Anyway there is a good documentation to implement these configuration.
Step 3) Integrate the sonoff for a light bulb switchable from multiple points is a little bit triki.
Many people around the net have found different way to interface these option and some other do not have clear ideas about it and I wanna try to clirify these point.
The main idea is to add 2 wires (depending on your specific configuration) to be able to do what shown in following pictures
Because the switches lines before the modification was working as I/O, I didn care to understand deeply how it works in the middle, I just need to undestand which is the first and the last…I hope pictures will be exaustive.
Anycase I suggest to think about how to restore the original configuration in case of needs.
These is my experience with these fantastic objects, smart home, smart price!