Yeah, it's kind of intimidating at first, but it's actually pretty basic. Just don't over think it.
Essentially, pretty much everything is earthed to the chassis/engine block, then just requires a live feed from the battery to make it hot.
I will be running a thick earth cable from the block to the chassis, so both the engine and the chassis will have a good solid earth to the battery at all times.
When it comes to wire size, the more power needed, the thicker the wire needed. Don't cheap out on wire size.
I've not got into relays yet, that's a whole other can of worms lol.