Prepare work surface by covering with newspaper, cardboard, or large tray. Position paper cup on work surface. Remove metal cap from 1 clear plastic ball ornament.
If using glitter, sprinkle some inside the opening of the ornament.
Squirt a bit of each color of acrylic paint into the opening of the ornament. Slowly move ornament around to mix paint and coat inside of ornament. Add more paint as needed.
Once entire ornament is coated, prop it upside down on top of paper cup, allowing excess paint to drain into cup. Leave overnight or until paint is completely dry. When dry, add metal cap back on.
If desired, decorate outside of ornament with Christmas stickers, rhinestone stickers, and/or 8 inches 3/8-inch Christmas ribbon for hanging.