You should abandon the idea of discussing the subject in the form of one big chat as you will overwhelm your child with more bewildering and distasteful information than she can take in at once.
Instead, think of it as a gentle conversation that will take place over a number of months. Keep your explanations as simple and specific to the discussion as you can. Don’t offer further explanations than your child requires, an eight-year old asking what “birth control” means is not necessarily asking you for the full mechanics of how sex works.
One of the hardest things is not letting your child see how embarrassed or uncomfortable the topic makes you. Try to remain calm and respond to the question in a natural way. This will let your child know from the onset that they don’t have any reason to feel nervous about asking you questions.
When you find yourself faced with being asked to give a technical description of intercourse, it might help to begin by saying “This will sound awful now but it will seem different when you are older".Then give a straightforward and honest approach such as “When a man and a woman are in a relationship and decide they want to do this, the man’s penis goes inside the woman’s vagina, sperm comes out of the man’s penis. Sometimes the sperm meets one of the tiny eggs inside the woman’s body and that egg then grows into a baby. This happens in a woman’s uterus.”
Once you make it through this, you should expect your child to be shocked and suspicious. Don’t be surprised if she suddenly changes the subject, walks away or acts as though she hasn’t heard what you have just said. She has heard you; she just needs time to absorb the information.