I wish time travel was real. Just to go back and keep my interest in Satanism to myself. Instead of revealing and broadcasting it to the world.
Once you tell anybody that you are... You can't Ctrl+Z it!
Still, the best way to introduce someone to Satanism is through The Satanic Bible. Probably, you would want to test the waters by throwing some questions or discussing Satanic themes with them. However, these are not guarantees that the person is receptive.
Satanism is not how the majority of people think. It makes them uncomfortable and someone who has lived most of her life within the "god-given" religions may not be very receptive.
What do you gain from her learning about your Satanism? What will she gain? I think you should think thoroughly about it before you reveal who you are.