Ming from Agents of Speech teaches three core techniques for helping non-verbal children speak: capture their attention first, establish routines, then teach sentence structure matched to context. He also recommends adding written words, gestures, play, and frequent feedback.
Chapters
Things that will help non verbal children talk. And this is the same thing. As we go further down into helping children to make sentences, same thing. We get their attention, we create a routine. We tell the sentence structure, the linguistic elements that matches the context. We add like written words, gestures, context, and play. That makes the sentences, the phrases that you're saying, come together. And a lot of times that comes as a form of feedback that we do a lot. And after you have these three things, you can apply it to children of any language level. For non verbal children, the first thing, the interactive part, is the most important part. If you don't get them to engage with you, you can't do anything.