How to wear Petticoats?

Hi,

I have just bought my first petticoat, but I'm not quite sure where to wear it on my body. I have a high waisted skirt and if I wear my petticoat above my belly button, then it makes the skirt scrunch a little. My skirt is fine if I wear it just below my belly button, is this where you are supposed to wear them?
Also is it the general practice to sit on your skirts, or let them fluff up around you? If I don't sit on it, then my peti starts to show a little.

Thanks. :)