I don't know either. She kept it very secret from us. She seemed to learn quite a few words off by heart initially, over a year or so, and also was writing things she was copying etc; and then seemed very unconfident that she was actually reading so we suggested a reading age test for her, which came out as older than she was (wouldn't work for an average age reader - mine was ridiculously young for an autonomous learner!). That gave her a huge boost of confidence and she suddenly wanted to read to us, but not reading scheme books - she read those 'read by yourself' ladybird fairy tales that have four levels. She whizzed through the levels over about 6 weeks and then suddenly stopped reading to us again. And then we realised she was a very fluent reader. Now, aged 7.5, she spends hours upstairs reading chapter books from the library 
I think she's exceptionally young to do it like that, and I can't see DD2 doing it anything like as young, but the beautiful thing for me was that she did it herself.
IME of other HE'd children, when they want help they ask for it. YOu don't need to sit down and say 'now we're doin greading practice' unless they ask you to do that for them.
other than that just lots and lots of reading together, playing on computer games (DD1 learned to read words like 'games' and 'play' and 'loading'
), playing any games that require a bit of reading - even if you're the one doing it. Lots of writing too - writing lists together, birthday cards, menus for pretend cafes, names, notes to eachother. And lots of playing with letters - those big foam jigsaw mat things are fun; and rhyming games.
There's a great book called 'Read With Me- An Apprenticeship Approach to Reading' by Liz Waterland that I recommend to loads o parents. It's out of print but you can pick it up really cheaply on Amazon usually.
Other good books I've found are the 'games for reading' and 'games for writing' by Peggy Kaye.
I agree with SDeuchars too - if a child's ready to learn to walk, they'll do it. If a child's ready to learn to potty train, they'll do it. They may need a little assistance but we trust them to ask for it and go with them. Then suddenly we decide that they dont' know their own needs and abilities and readiness anymore and we have to impose onto them our own schedules and teaching and I think that can be really detrimental.
I think letting achild learn to read autonomously can be quite scary until you've seen one do it, particularly when they learn really 'late' compared to their schooled peers, but I think you just hav eto trust taht they're learning at the right age for them.