I would read a short bit, then say, “Tell me back what I just read,” then read the next short bit, then say, “What happened next?” and so on. I would start with just a paragraph at a time, maybe an entire page at the most, not a whole chapter. This does two things: your children only need to pay focused attention for very short periods of time (this may be a new skill for them), and your children only need to tell back a short bit at a time (this is certainly a new skill for them). Keep your questions simple, especially at first. And realize their narrations may only be 1 sentence long at first (especially if you are narrating between paragraphs). When you are done with the reading, you can have more of a “conversation” about the reading – what the children thought/liked/disliked, NOT what you are itching to say 🙂 But I wouldn’t even do that until well into the school year when narration is normal. To start, I would only read/narrate 10 minutes from any one book at a time, even if that means stopping in the middle of a chapter or an exciting part of the story. Just make sure you always begin every day’s lesson with a “where did we leave off” type of question. As the months go by, you can increase slowly up to 20 minutes of reading/narrating at a time.
Narration will become as easy as breathing – they probably do it already with certain aspects of their life (exciting stories they are reading, exciting things they are doing or saw someone else do), so I would incorporate it right from the start, but keep your expectation low at the beginning (for yourself as well as for them). If I remember correctly, it only took maybe 1-2 weeks for us to feel comfortable with the whole thing (narrating from maybe 2 books a day).
That said, I would NOT require narration from literature books – read a full chapter aloud unless it is a long one, then read a half chapter. We usually read literature books during lunch or dinner time. But I still always ask “what was the last thing we read” before starting in on a new chapter. Keep it simple!!! It will feel normal soon enough, but only if you do the hard thing of being uncomfortable for a short while. That’s my 2 cents 🙂