Just tell her exactly what you want to achieve (showing samples might help)
Make sure that you do so before she arrives for the shoot, i/e when she confirms that she would be interested in shooting with you.
Don't be afraid to use clear language, (topless, semi/full nude, full frontal, open leg, etc.) make sure that she lists the style you want to shoot in her preferences.
The fact that a model has that type of image in her portfolio doesn't mean that she is open to shoot that style with you, always double check before booking her.
Hope it goes well
Exactly. Don't over-think it...just be relaxed and professional. They can tell almost immediately that you are not a creep...so just be relaxed, be yourself, and go about it as a gentleman does.
Unless you are doing a full nude (naked) artistic type shoot, then the term "partial nude" goes a long way, especially if you just say "lower partial nude" or "exposed behind" or "topless"...that pretty much is enough description to get started with dignity, and allows the model some room for her own comfort and interpretation. I don't like the "clinical" words like Vagina, and I just feel too damn silly saying things like "cootchie" or "cookie". Just be dignified and diplomatic. It will work out.
Have fun! A relaxed easy-going attitude is contagious.