What you need:
Roll the open end of the pillow case a few times to make the "bag" a bit shorter. You can tape this from the inside.
Use your Halloween cookie cutters to stamp shapes onto the pillowcase. Sprinkle with glitter before the paint dries.
Once the paint has dried, add faces to your pumpkins, ghosts, or other shapes.
Take this bag along to collect candy!
Another tip would be to paint using fluorescent paints. This would be great if you are planning on being out in the dark!