Wiring the Dash
The wiring for the dash is one of the more complex parts of the build, that said, if you consider it to be a number of individual circuits then it can become fairly simple.
The gauges are very simple, each has a 12v feed an earth (negative) to power them, they also have an illumination feed that is triggered by the lights being put on . They also have a signal feed, usually just one wire, or 2 in the case of the speedo.
To greatly simplify the wiring for all this, all the gauges are daisy-chained together sharing the 12v feed, the earths and the illumination feeds, that means you only need a couple of wires entering the circuit to power the illuminate the entire instrument panel.
The switches will generally be much the same, sharing earths and having feeds which are switched, such as to and from the fog light, heater and brake fluid level test switch.
Below shows a mocked up dash with the basic wiring for the gauges, the black multi-way connector with pink wires contains all the signal feed wires, the only others are the read and black power circuit and the red/white illumination feed, that's it!
Once you've got all this in place, its a very good idea to add a number of multi-way connectors rather than use the vast number of single spade connectors that come with the loom, this will make getting the dash connected and disconnected far easier.
Here is a completed dash back with all the wires in convoluted tubing ready to pass the SVA. (there are a even few extras on this dash that can be ignored such as the additional electronics box)
Once you have got the basics sorted, you may decide to add some additional circuits such as Radiator fan overrides, security systems, interior lights, engine start buttins etc. This is all very simple once you have the main dash wired up.