My DD is 4... like pp said, It has never been a secret but I also don't push the converstion. I did make an adoption story book (included first , foster and forever family pics) and have read ( paraphrased versions) of it to her a couple of times. She is proud of the book and gladly shows it to others but shows very little interest in it otherwise.
So I agree "now" is the time. DD (4) has started asking some very simple questions..
where did u buy me?
where did I come from?
Questions our difference in skin / hair color?
etc..
I am the kind of person that "overthinks", "over-analyzes" and over explains
But I agree with previous posters.... sometimes the simplist explanations are the best and answer her questions as they were asked.
I have made b-family contact and hope to travel to visit her b-family in the future. So I do feel I need to step up the conversations so she is prepared. So like you wish I had a magic handbook on how to handle this!
Good luck