Intro: Easiest Arc Reactor.
Not really a step-by-step, just an idea that I'm gonna do for Halloween.
Step 1: Buy.
http://www.amazon.com/SE-Camping-Travel-Emergency-Light/dp/B0020MZ70O/ref=sr_1_74?s=hi&ie=UTF8&qid=1366759665&sr=1-74&keywords=led+tap+light <- new link, sorry the old link broke.
And a balloon, standard party size.
Step 2: Make.
Cut the mouthpiece off a blue balloon, about an inch up. stretch this over the top of the LED light, tight enough to where there isn't a big bulge, but loose enough not to push the center button without your help.
Step 3: Wear.
The easiest is to tape it to your chest, like I have in this picture. It hurts a little. Another idea would be to use the hook on the back of the light, and wear it like a necklace. Or you could do what other people on Instructables have done with theirs, and sew a pocket into the inside of the shirt. Good luck, and have fun!