We didn't do much preparation because DD2 was so unhappy at school it didn't take us that long. However, I did discuss with her things she thought she would miss.
Her main worry was (it was October) she would miss the school Christmas dinner, party, etc. It's funny that the things that seemed big to me, like friends, didn't bother her at all, but the things that seemed small, like "I won't get my breaktime fruit" seemed to matter most.
We unpicked all the stuff she would miss and talked about how we could replace those at home with something similar or better. Then we talked about things she would like to do that she couldn't do whilst she was in school. Once again, things that seemed insignificant to me were big draws to her. She accepted that some things weren't really replaceable like for like, but that in balance, home came out as the much better option. She was 8.
Then we told the school mid week and she came out on the Friday, to give her chance to say a quiet goodbye to her teacher etc.
It was the best decision ever. Over 4 years later and she's loving it, and has been joined by her older sister. :)