How to Map a Boundary?

I am new to Avenza Maps and would like to map the boundaries of the wiring for our Invisible Fence dog layout.

The IV company recently installed underground wiring and flags every 5-10 feet. Our dog has been trained NOT to go past the visible flags. She hears a sound when she gets too close to the underground wiring and flags. In fact, she stays several feet from the flags. The flags will be removed soon and I will start mowing the lawn including areas where the flags are now.

Once the flags are gone, I want a map that tells me where the underground wiring is. I'm thinking an Avenza Maps will do that. What's the best way to learn how to create the map?