Helena wears a black fitted dress with ruffles poofing out along the top and a red sash sewn around her hips. Under the main black fabric is a layer of red that pokes out of the solid black.  Under both the red and solid black is a layer of sheer, stiff black fabric that pokes out along the bottom hem.  The main solid black is either drawn up or there is a slit on the left side of the dress to reveal the red underneath. I'm pretty sure that they just grabed the bottom part of the dress and stitched it up above the sash. The dress zips up in the back with an invisible zipper.  There are long skiny triangles of sheer black fabric sewn over the sash and that lead down part of the dress.  See the close-up shot of the sash.  To make it poof out towards the bottom, you can either buy a slip that has mesh-like fabric attached in layers at the bottom and wear the dress over that, or you can sew the fabric into the dress.  With prom dresses they are usually two pieces, so I would recommend doing that.  Since this is a strapless dress, you could sew it along the top of the dress or attach it along the waist before attaching the sash so the stiches won't show.  You'll need to buy a black one so it matches the dress.  If its white it may be noticeable and you'll have to do extra work to cover it up.  The bottom hem of the dress falls just below her knees and goes to her ankles in the back.  You can still see the red poking out under the black.  She wears black ballet flats that tie up with ribbon around the ankle.  She also wears a veil that falls down to her eyes.  I think it's attached to a headband (or something like that) to hold it in place.  Add a bouquet of roses and voila!