My 3yo asked me to teach her to read and write.
Reading, we started with finger under each word then every time 'a' was in a sentence, I'd pause and they'd say it, then came 'I'. Next was two letter words introduced one at a time, 'it' then 'to' then 'at', then three letter words introduced one at a time 'and' 'the' 'boy'
Then try the names of the characters in the book.
Writing, again, start with the things you're reading, a and I and it etc. you can get books to trace the letters so it's the same way they learn to write at school.
They don't need to know many of those words for them to feel as
though they are reading most of the story themselves.
In our case, it seemed to happen by osmosis, but all children are different. Mine was really interested and wanted to do it because it was fun and they felt a sense of achievement, then I was led by them when they wanted to do more.
I was never taught to read but I could, well before starting school. I suppose it's something that just clicks for some kids, but I'd never ever make a young child sit still and learn, mine wanted to, others the same age would rather run around.