The only tricky part about piping royal icing ghosts is getting the consistency of the icing just right; you want it to flow in a ghostly fashion, but you want it to be stiff enough to hold its shape. My icing wasn't thick enough, so I squeezed it out into a bowl, mixed in some stiffer consistency royal icing, then refilled the bag. In addition to white royal icing, you'll need a petal tip #104, parchment paper squares, a flower nail, a glue stick, a Styrofoam block to rest the nail in, a black food-safe marker like the AmeriColor Gourmet Writer, and a dusting pouch filled with a 50/50 mix of cornstarch and powdered sugar. Start by sticking a parchment square to the flower nail with a dab from the glue stick.
When the icing has firmed up, draw in some eyes with the black food-safe marker. Peel away the parchment and use these ghosts as cupcake toppers (or perhaps in the graveyard scene inside a "panoramic egg" eyeball).