Blacklight posters are decorations with fluorescent inks that respond to ultraviolet lights. Fluorescent ink absorbs invisible ultraviolet, or "black" light, and re-emits it as a bright, visible colour. This makes it look as if the poster is glowing when the blacklight is turned on. Many different craft paints are fluorescent, so you can make a blacklight poster in pretty much any colour you want.
Buy stencils or create your own. If you want to make a stencil, glue or tape the picture of your choice onto a piece of cardboard and cut out the main shapes.
Trace your stencils onto a poster board. Use a normal graphite pencil, and trace it with just enough force to make a light line.
Go over your lines with permanent marker. This will make a bold border between your images.
Get some poster paints. Turn off the lights, turn on the blacklight and look at them. The way they look in their pots is the way they will look on your poster.
Paint your shapes, using one colour for each shape. Then, paint in the background in a fairly dim colour.
For a really wild effect, use invisible UV paint. It looks clear in normal light, but glows brightly under blacklights. See "Additional Resources," below, for more information on UV paint. You can get blacklights as fluorescent bar lights or as incandescent light bulbs. See "Additional Resources" for more information on blacklights.