I am trying to add ECW support into GeoServer 2.12.1 but no luck so far.
Does anyone have a step by step guide to add the ECW support?
Step-by-step instructions for installing ECW support are provided in the GeoServer manual, as part of the GDAL Image Formats section.
I'm not sure if its a problem in your case, but for some reason people miss the part that says:
Moreover, in order for GeoServer to leverage these libraries, the GDAL (binary) libraries must be installed through your host system’s OS. Once they are installed, GeoServer will be able to recognize GDAL data types. See below for more information.
Also note that the ECW licensing contains some non-standard conditions, which you should evaluate for your specific needs and situation, or seek legal advice to interpret if needed.