If you have the car stripped for restoration it is best to wait until most of the rebuild is complete before doing the wiring. Painting and fitting parts is easier without having cables to worry about. I restored my 12 last year and used an Autosparks loom. This was made more complicated because the loom was modified by Autosparks for flashing indicators and an alternator. As such, the wiring diagram in the workshop manual was only partly correct. However, assuming that yours is more standard, it is not too difficult to get the wiring sorted into the right places. The first thing to do is lay out on the floor the component parts of the loom you have bought. From memory, there will be a small set of wires for the dashboard clocks and switches, a main loom that goes from the dashboard to the fuse boxes and regulator box plus two other bits of loom that do the rear lights and front lights and horn. I seem to recall that I had to source the wires from the dip switch and horn to the junction box near the steering box. Then to make sense of this lot you will need to copy and enlarge the diagram from the manual and then identify each wire and label it. I did this with masking tape and noted what the wire was for and if it was, for example, the switch end or the fuse box end. All the wires are colour coded so it's just a matter of working through them. Once you know all this it's quite easy to lay the loom in place in the car ready to connect at the junction boxes, fuse boxes, switches and components. At this point you may feel confident enough to get the soldering iron out and start making the connections or, alternatively, engage an auto-electrician to do it. However, the job will be made easier and I assume cheaper, because the cables will be marked and in place.
I hope that helps a bit but I do recognise that it looks baffling to start with!