For the Dynon system, I built the serial wire harness and did continuity check for each of the pins. This is the easy wire harness to build and test since all of them will have the same 8-wire pins. The only serial harness that has 9 is the EMS.
For the rest of the wires for the transponder, radio, intercom, I built most of harnesses and tested the EFIS+units while they are on the bench. I was a complete beginner at this so having everything on the bench helped me to debug. It turned out, most everything went together well the first time. The only problem was the wiring for the intercom, with the multitude of solder connections, and complicated wiring diagram, I had to redo this wiring after discovering I did not have radio com.
I strategized powering up the different units individually and incrementally connected more units to the main EFIS. The main EFIS only need 2 power wires and ground wires. You get the "X" on the screen but as you add more stuff, like ADHARS via the serial cable, you start to see the attitude indicators. This worked for me. I remember the sequence was EFIS, +ADHARS, +GPS, +ADSB, +RADIO, +INTERCOM. The intercom was last since it was the most complicated for me wire.
After all the avionic units could "talk" to each other, I mounted them plane. The EMS wiring was completed on the plane since the wiring needed the engine and most of the avionics in place. One mistake was I didn't have the service length as much as I would like. It was enough but I rather have more length.
Some of the pros did all of this with everything connected to the panel, and did the wiring while sitting inside the cockpit. I could never have tried since my chance of making mistakes will go up exponentially.
I planned one month for this task but it took me more than 3 months since I was trying to learn the wiring diagram and the airplane electrical system.