A.S.
When i was that age my mom had teh talk woth me. BUt it was more like a short conversation that was had over the course of years. I don't think you need to put that much pressure on yourslef. At this age just answer her questions as she has them. i know that i was in 3rd grade when other kids at school started to talk about sex. I remeber on girl saying she found her paretns in there bed room "stuck together". When i asked my mom what she was talking about she just told me that there were having sex which is something that adults do when they love each other very much and i did not need to worry about it for a very long time. At 8 that worked for me. If you want to perpare yourslef get some books from the libary that are age apporate and memorize the answers. I think as long as you are open to talk to her she well come to you when she has questions. Also you can bring it up when you see something on tv or in a magazine that she is interested in.
Good luck and don't stress to much
A.