try googling "crinoline".look in the images..this is basically a separate skirt the has layers of gathers like a twirl skirt. this gives a lot of poofiness, especially if the hem has two layers of gathered tulle.
i reccommend a separate skirt because it is easier to remove to wash the dress.
if you shorten the apron a bit, like maybe put a tuck -about 2 inches, then the black skull fabric will pop out and the "apron" will look more apron-y. another idea, you can put black ricrac along the apron sides.
honestly, this looks like something from matilda jane!