This is a quality item designed so that it's easy to fit. 

First you fit a standard single box on the wall. Then fix the back plate to it.

Wire the socket & clip in to place. Once all is working correctly, add the chrome plate.

This avoids wires being trapped as you push the face plate in to place.

I have also used these as a slave by just fitting wires 2 and 5 (no ringer)

Regards,

D