Cut an old t-shirt so that you have a square piece of white fabric big enough to fit over your glass.
Turn the glass upside down and drape the fabric over the glass. We used one made out of glass, but we're sure plastic will work also.
Pour some glue into a separate container (we used an empty baby food container for this messy part). You need a good amount of glue. Add a little bit of water to thin it out. You want to be able to easily stir the glue with a paint brush but still want the paint brush to be coated white when you pull it out of the mixture.
Paint the glue mixture onto the fabric as it sits over the glass. If you don't get to the very edges of the fabric it is ok. The parts that aren't covered in glue will be more "flowy" than the parts that are, and the bottom of a ghost is supposed to be like that!
Set the project aside to completely dry.
Once the fabric is dry, peel the fabric off the glass (the glass is not part of the finished craft). Have an adult use a knife or a pair of scissors to poke a small hole in the top of the ghost and thread a piece of yarn or string through it (knot it underneath) so you can hang your ghost up.
Use a marker to draw on a face.
Hang up your ghost for a less-than-spooky decoration.