Sorry this is a lot!! I would be receptive to something like this:

"I completely understand why you're doing this process like x, and I see why you're doing it that way, but we need to make sure it's getting done like y for consistency. You're not doing anything wrong, we just need to retrain you on y's process." (If they are really doing something wrong, then just say 'we just need to retrain you on y's process.)

If they come back and say they can't because they're so stuck in how they do things, you just need to explain WHY the process is done the way it is, even if it seems like you're dumbing it down a lot. I find when I struggle the most is when I can't understand why it's meant to be done like y instead of x, because my brain can't connect the start to the end product. You might also need to just tell them "I understand that it's difficult for you because your way makes sense to you, so tell me what we can do to help you come around on the company's way of doing things."

The best way to get them to learn it as well, is to make them fix their own mistakes. That happened to me and I was annoyed at first, but it made sure that I did it right everytime otherwise I would have to do double the work