The wiring is pretty standard lines A and B (signal) which comes into your property connects to the master socket (usually) terminals 2 and 5. The wiring then goes from the same two master socket terminals to the correspondingly numbered terminals on each extension.
In addition to the above, there is an additional wire (ringer circuit) that runs from the master socket (terminal 3) to the correspondingly numbered terminal (again) on each extension.
Caution! if the box is fitted with Insulation Displacement Connectors (IDC - which look like metal slots) - don't use a screwdriver to push the wires into the slots, as these are easy to damage - best to get a pukka IDC tool from your local B&Q type store. They're not expensive and will save you a load of hassle. In fact it's as easy as positioning each wire over the appropriate slot and pushing down on the tool - simple as that. If you view the links already provided on the previous threads then you should be okay.
Commiserations on them being BT and not VM - but, sigh......! it's better than nowt I suppose.
That's it and good luck.