Well, she shouldn't be working to understand PhD level 2-ump vs 3-ump mechanics. She should be working to finish grade school and high school level stuff.
My point is "somewhere at the beginning". That somewhere will differ by individual. My son at 10 years old knew the concept of tagging up and knowing when to run and when to tag. And understood the basic concept of the IF. We have 10 year umpires who screw up force plays.
If you were teaching her to be an auto mechanic, you might start with how to change a flat tire. My daughter needed help when she got a flat. And then maybe an oil change, filling the washer fluid, changing wiper blades. It depends upon how comfortable an individual is with what's being taught. (Then she'd turn into Ms. Vito in "My Cousin Vinny".)
I recall a couple of SAP software implementations at 2 different companies that I worked at that was like trying to drink from a fire hose. Neither implementation was very successful with people making a lot of mistakes. And the training that was provided was what I'll refer to as "sunny day scenarios". We didn't have alternatives for when something didn't work properly. How should we "undo" it, could we ignore various error messages, etc.
Since your daughter played the game, that is a definite advantage. Getting her behind the plate will likely be a bit stressful for a while. Lots of stuff to learn and remember.
If there is a junior league that she could work games, that could help.
One more thought is to get her a mentor (other than yourself) to help with her training. Sometimes it's not good to be discussing what things didn't go exactly right on the ride home. Just sayin'. You will become the veteran ump she'll go to with questions and scenarios.
And maybe get her an account on Officiating.com.