(1) 100-200 small black plastic spiders. Flat back spiders work best or you can cut off the ring part from spider rings.
(1) Roll of white self-adhesive shelf liner (self-tack).
(1) Small handheld glue gun.
(1) Sharp pair of scissors.
(1) Small strobe light.
#1 - Begin by completely covering your ceiling with the shelf liner. This helps minimize the glue taking off your paint later. The first year I did this I just glued the spiders to the ceiling. Great effect, bad idea when taking the spiders off.
#2 - Taking care to apply as little glue as possible begin gluing spiders in various patters about 3 inches - 5 inches apart across the entire ceiling. Make some look like they are following a leader, group a few other like they are gathering to attack.
#3 - Add a strobe light and let the creepy crawly begin.
This creepy room works best in a small room like a bathroom but I can imagine a larger room might be just as creepy if you add-in a bunch of larger spiders too.