First off to fit the NTE5 ADSL faceplate splitter you need to remove the front lower half of the NTE5 :-
You could discard the front lower plate but keep it just in case! :-
If your modem or router plugs into your master socket and you have no extension wiring all you need to do is plug the ADSL faceplate in! :-
If you have extension wiring for phones only then you have probably purchased the 3 way version (https://www.run-it-direct.co.uk/BTNTE5ADSLfaceplate.html) , to connect extension...