Vans basic electrical system diagram
In My RV-6A preview plans book (I'm not sure of the name) there is a basic electrical diagram that I used as a starting point. That establishes the interface of the battery, alternator, master relay, starter solenoid, etc. If you can study that and get the essential needs of the system prior to electrical distribution to the rest of the airplane's electrical needs you have the conceptual hard part taken care of the rest just becomes a task of supplying power as required by the lights, avionics, with circuit breakers or fuses to protect the wiring. I have a 60 amp CB in the panel at the head of the distribution then appropriate level CBs for the power requirements of the individual elements down stream. I have and avionics bus enabled by an avionics master switch and other applications that parallel (do not go through) that switch. Every wire must be calculated to get the right size (Gauge) based on power and distance (signal wires can just be the size specified in the hardware manuals - 22 gauge, etc.). If you just set down with a drawing board or a piece of paper on your kitchen table, copy Van's basic diagram for the power source and draw a bus from which the power is drawn to boxes with the pins as defined in the equipment manuals or to elements such as lights through appropriate controls such as switches, dimmers, circuit breakers it will all come together nicely along with design comprehension. Documents available from the FAA tell how to determine wire size needed for each application and documents such as Aircraft Spruce catalog tell what equipment is readily available for implementing your design with aircraft quality hardware and wire. This drawing is critical for the maintenance of your electrical system as you will not only have to make repairs but you will want to modify the system after the airplane has reached flying status. Constraints of the airframe will force a lot of implementation decisions as you wire it up. Mine started out bare bones basic IFR capable. I added an autopilot system after a year of flying, 4 cyl EGT, 4 cyl CHT, a second nav radio, a power outlet for a hand held GPS, etc. after that.
Good luck, it is a lot of careful work but it is worth the effort.
Bob Axsom