Okay, I'll admit, this one requires either a lot of money, or a lot of skill to pull off... But seriously, the Vashta Nerada had to be one!
The number of viewers seems to grow each day, that's good... I think.
And here is the materials list:
Lighting equipment (or a flashlight, depending on how smart your victim is)
A gullible Who fanatic
OH! Maybe some sound effects could be good.
1. Set up the lighting and sound equipment in the areas you wish to use them (for best effect, use a library)
2. Have people with some skill using the equipment (or strategically placed with flashlights)
3. Move through each room with the victim, and maybe move through an area that is darker than others. Upon exiting, have lighting people make it appear that one friend (not the victim) has two shadows.
4. Have one person stay behind with the person who has two shadows, and then run later after the person screams
5. Repeat the process until only the victim remains.
6. If you really want to scare them, make it look like THEY have two shadows just before ending it.
Poor quality, A smidge, I couldn't make this not sound lazy, so apologies.