Q. When do you explain the TPRS method and how?
A. I don’t explain it. I just do it. And, if I were new to it, I would spend a lot more time in the grammar book than in tprs at the beginning, just to make them want more stories. It is better to make them want more and to “get” how bored they are in the book (by comparison to the fun comprehensible input) instead of doing a big “Hey kids we are going to do something new and exciting this year!” thing, completely alienating parents who believe in the book, because then you would be put on the defensive. No teacher who does TPRS properly should ever feel on the defensive with anyone, any more than a doctor using the newest healing methods should be forced to explain why they use them to people untrained in medicine.
In my view, teachers new to TPRS need to make the kids plead for more stories or PQA. So I am advocating that they do like five or ten minutes of TPRS every few days to start. Don’t give away your gold too early until both students and parents can appreciate what it is.