In a nutshell...go donate blood at a one of those buses that just drives around and parks at shopping malls etc...act confused about the ENTIRE process. I mean, like you have NO idea whats about to happen. All's you know is that you're gonna get some free cookies at the end....when they hook you up and your blood begins flowing into the bag for a while, pull out a real blood bag and tube, except its full of some type of fake edible blood. Put the tube connected to your fake blood bag in your mouth and start drinking your own "blood" because that's what you thought you were supposed to do. The folks are really gonna freaked out and be grossed out. Hide the real blood bag under a coat nearby. Use the confusion of the tubes, and the empty bag of fake blood to convince them. A partner or two is necessary for distraction. One partner would be better because that leaves more people to be pranked. Stumble off the scene looking for some god damn cookies....my name is Tyler Ervin, and I have the Jackass logo tattooed on my buttcheek, and its about the size of your fist.