It really is simple!

Each wire has a colour & number associated with it as below.
In reality only 3 wires are needed wires 2 , 5 (BT Line) & 3 (Ringing wire)

Either IDC terminals are used (Insulation Displacement) or screw terminals.
The master or NTE5 socket is supplied by BT or the cable company.

The sockets are wired as follows - remember that pin 1 connects to pin 1 , pin 2 to pin 2 etc & you only need to wire up pins 2 & 5 & 3 on a domestic installation.

This picture below shows the IDC connectors and components of a Line Jack Unit 3 (LJU3/1A) Master socket

This picture below shows the IDC connectors of a Line Jack Unit 3 (LJU3/2A) Secondary socket

The usual caveats apply - don't try this if you are unsure. Do not connect direct to the BT wiring - the NTE5 is designed as an isolating unit - BT wiring on one side - customer wiring on the other.

And finally if it doesn't work don't blame me - it's all your own fault - this is for information only.