Do you put BC under the fondant or nothing under the fondant when covering dummy cakes? Anything else you recommend to do with dummy cakes before using them for the first time?

Are you using the soft foam type dummy or the stiff styrofoam? I have heard that on the soft one you can dampen it and then apply the fondant. For the stiff type, you can use a thin layer of piping gel and then cover. I do think you have to have something under it for it to hold better.

Make sure you lightly sand the upper edges first on the foam. (corners as well if square) Lightly spritz with plain water and apply fondant.

I only make faux cakes and this method is no fail.

I've misted mine with water, and recently tried covering with crisco. Both methods work well. My first try with nothing was awful, so I got these tips from CC

I've tried several different things and water works just as good as anything else with a lot less mess. But on your styrofoam, do be sure to sand the edge slightly as dsilvest recommended.