I've downloaded EasyCustomLabeling-2.0 but I don't know how to activate the plugin or where files should be placed.
It looks like the latest version was ported to the next version of QGIS, which none of us are running.
You should replace it with the previous version:
Depending on your operating system, installing the plugin involves copying the un-zipped folder here:
C:\Program Files\QGIS 2.18\apps\qgis\python\plugins
or check this for a Linux version:
You might have to go into the Plugins menu and 'enable' it.
Then you'll see this icon on your Labeling toolbar: