Probably all the major equipment vendors have installation manuals that you can download. Those will tell you the external switches, power and ground requirements for that component. Then "Aeroelectric Connection" by Bob Nuckolls is a good guide to the general electrical distribution system and the various components that comprise it. It's available here.
But even at that, there are vendors that offer a sort of "black box" version of the distribution system so there are more alternatives.
Here on VAF, there are frequently schematics offered for comment, and you can learn a lot from those. A search will find several, and there is an entire Electrical section here too, that's worth browsing. If you need specialized equipment such as oxygen, there are some vendors for that stuff too. I've had really excellent support from Mountain High Oxygen, for example, but just about every equipment vendor has good to excellent service. If you find otherwise, then that's another consideration.
Besides Aircraft Spruce, there are at least a couple of good to excellent general-purpose aircraft electrical parts vendors. Browse further through VAF and you'll find recommendations. Some of them might offer to build entire instrument panels and wire them up. I've seen one for an RV-10 and the fabrication quality and overall documentation and packaging was superb. Bear in mind that approach is not the most economical, although the quality is usually top notch.
RV-3B still workin' at it