Hi Missy, well you put a fairly thin coating of buttercream underneath, enough so that the cake doesn't show through, that is for sure, but not too thick. You make certain that it is really smooth because any bumps in your cake of buttercream are magnified once the fondant goes on top.
Generally with regular rolled fondant, you are supposed to use this at a thickness of 1/4 inch thick. With the marshmallow fondant well people use this a lot thinner. I have heard that you should still roll it out to 1/4 inch, but others like it very thin, so there you go.
With the regular rolled fondant, well you want to make certain that the buttercream doesn't crust or set before you get your fondant on there, so that it can adhere, at least that is the old standard way of doing it.
With the marshmallow one, well people do various things, some let it set, others don't.
I don't use the marshmallow fondant for covering cakes so I will let others answer.
Hugs Squirrelly Cakes