There's a documentary called "Spellers" about teaching nonverbal individuals how to communicate. Very life changing. God bless.

Honestly, I would go on YouTube and find a beginner course on sign language for autistic children. Our kids sometimes need to learn different words first that are important to their life. If your child loves cereal or crackers, you can learn those first because it’s vital for them to be able to communicate specifically. If your child makes certain movements for something specific, make that your personal sign for whatever it is. They will start seeing that when I do this, I get that. It doesn’t even have to be hands and arms only. It could be a certain noise, shaking the body like a dance move, or spinning in one circle. In the beginning, it doesn’t all have to be professional. The key of teaching any type of language is communication. The same way that babies figure out saying mommy, or daddy, gets the attention of the person they want. If anyone babysits, just make sure you leave them a note of specific words, and maybe make a short video showing each sign slowly 2 times. You could also make a list to hang up with pictures of the sigs for each word.