You know, if you talk about sex with her directly, she may get defensive. How to mention something else and gradually draw the topic to sex so she won't feel it is unnatural so she will be receptive and feel natural to talk about sex with you. How to do it?
By not being offensive about it. Some people (regardless of gender) will find the subject offensive no matter what you say about sex. If it is one of these people you are referring to, then the answer is, you can't.
Best time generally would be just after you have sex. Just remember to make sure to give her some kind of compliment as you start, makes us relax a little bit.
Don't avoid talking about for a long time, then it's just going to be awkward when you do. Speak your mind. If she gets offended that's her problem and not yours