When I built my RV-10 many years ago, like Carl, I documented each and every connection in Excel. I also created a tab for each avionic device.
The end result is that you can look at P1 on my GTN650 and see what pin each wire is connected to, the pin/connector/device at the other end, as well as any required software settings to make that connection.
Fast forward to today. I had intended making nice schematics after I started flying. It's been over five years and I still haven't got around to it yet. With that said, I have that Excel wirebook on a laptop in the hangar, on my iPad, and on my iPhone. I refer back to it all the time or when I need to make a change. It has served me well.
I'm not saying to not draw a schematic, just that sometimes simple approaches work well too.
bob
To build on Bob's comments... I opted to do my wiring diagrams on a "system" basis, with each aircraft system laid out on an 8 1/2 x 11 page of quad-ruled paper. My wiring diagram is drawn in pencil, old style. Zero cost, zero learning curve, zero probability of software incompatibility etc, and I never have to worry about a hard disk crashing! I then photocopy the finished drawing and scan the drawing. The photocopy produces a "hard" image that is placed into two binders, one that stays at the hangar, and one that stays at home for reference. The scans are on a USB stick that stays in the airplane and are stored on the tablet I use to run navigation apps so I have ready access while away from home.
Now what's this "system" approach? It's really simple and makes troubleshooting dead-easy. Let's take external lighting as an example of a system. The wiring diagram captures everything to do with external lighting, starting at the Main DC bus, through the circuit breaker(s), switch(es), wing root terminal blocks where, for instance, a single "NAV LIGHT POWER" wire splits to "LEFT NAV LIGHT POWER" and "RIGHT NAV LIGHT POWER", ultimately terminating at each nav light. Every connector and connector pin is detailed. When you have trouble with the nav lights, you are now troubleshooting from a single page diagram.
I've spent a lifetime troubleshooting aircraft electrics and avionics. So far this "system based" approach to wiring diagrams has proven to be the most effective.
Oh, one other point. VERSION CONTROL is important. Remember I said that I do things in pencil and paper? This allows easy editing. The drawing has a date box that is updated with every edit. Once edited, the new photocopied versions go into the binder, onto the memory stick and into the tablet. The previous photocopied version in the "home" binder gets a line through it and the word "Superseded" written across it, but it is kept in the binder behind the current version. This allows you to see the evolution of the system, from original, through to "as built" through to "as modified when I installed new equipment." It's a system that works very well and costs virtually nothing, while giving both a current view of the wiring as well as a full historical perspective of its development.