Our source here is ethnobotanist Wade Davis. In 1982 he visited Haiti to see if he could learn the secret of the "zombie powder" that local sorcerers, known as bokors, allegedly used to reanimate the dead. As told in his 1985 book, The Serpent and the Rainbow (a scholarly version, Passage of Darkness: The Ethnobiology of the Haitian Zombie, appeared later), Davis had little to go on but some tantalizing stories and a few contacts. Nonetheless, during his first week in Haiti he managed to meet two alleged zombies who'd been patients at a local psychiatric institute. (One would become the subject of a BBC documentary.) What's more, with the aid of a wad of greenbacks he was able to witness the manufacture of a batch of zombie powder. In a chilling passage he tells of a midnight trip to a graveyard where he watched a bokor and his assistants dig up the corpse of a recently deceased infant, portions of which--Davis is a bit vague on the details--were added to a witch's brew of plants, sea worm, toad, lizard, and fish.
As time went on Davis learned a bit about why zombies were created. Typically the victim had antagonized his family or neighbors, who hired a bokor to do him in. The bokor would spread zombie powder on the threshold of the home of the victim, where he would absorb it through his feet. After falling into a deathlike trance the victim would be buried then later summoned from the grave by the bokor, who would exploit the zombie as a slave.Read the rest here.
It also has a pretty neat and zombie related article on whether or not PCP turns people into cannibals. Remember kids, ignorance is bliss, but knowledge is power.