I went thru something like this with an ex. What I did was just did it the proper way and she imitated it. That way it wasn’t me telling her how to do something it was her looking at what I was doing and deciding I had the right idea. Stole (yes meta) this technique from the man who taught me basic cooking.

People naturally want to be competent and will emulate success. If you tell them that they are wrong they will dig in their heels. If you show them how to be right they will copy without losing face.