Voodoo (Vodoun, Houdun, Hoodoo, etc) is a hybrid of Catholicism and Africian religions, bred in Haiti where African slaves (of many differse regions and religions) were forced into the Catholic community. The confused slaves were now engulfed in a million religions when they were taught only one. What resulted was an intertwining of many tribal religions along a common thread. The slaves would call this new comingled religion "voodoo" from vodun, the west African word for Spirit. It is also beleived that the name comes from "vous deux", which means "you two" in French (the language of Haiti). This is thought to represent the karma-esque teachings of Voodoo, that everything you do to others will effect you, we are all connected.
It is estimated that voodoo is rooted in the Angolese, Caplaous, Congos, Dahomeans, Ethiopians, the Fon, Haussars, the Ibos, Libyans, Malgaches, Mandinge, Mondungues, the Nago, and the Senegalese.
It's central teaching is that God (as taught to them by the Catholics) manifests himself in the form of spirits, and these spirits can be called upon to help the living, as the living and dead are both part of the same whole. The spirits (Loas) will aid you in need, or harm your enemies, and because they have this power- it is important to the Voodun to honor them in ceremony- including dancing, singing, drumming, sex, and animal sacrifice (simaler to the white's "Rock concert")
Often in these rituals, the Loah will manifest themselves in the Papa (m) or Maman (f), the leader or High Priest/ess of the ritual. These "possessions" seem to place the possed into a trance and they rarely remember what happened.
Aside from the spirits of loved ones, the Loas (aka Sobo and Mystere) also include the voodoo serpant "deities" such as:
Bon Deiu -aka- Damballah
The supreme father, creator of the world. He
is "too wise" to speak, so when possessing someone, he usually sits quietly. Damballah is a personification of the Catholic "God".
Erzulie -aka- Ezili
the goddess of love, sex, and art. She is a trinity of Freda (virginity), Dantor (jealousy), and La Siren (motherhood). Erzulie Freda is the personification of the Catholic's "virgin Mary".
Agwe -aka- Agoue
god of the sea, sailing, and fishermen. He is the personification of St. Ulrich.
And a much large pantheon. Rituals of voodoo involve calling upon the spirits to do your bidding, not dissimaler to praying in Judeo-christian religions. Invoking the spirits and appeasing them often requires a sacrifice from as small as burning herbs or candles to spilling the blood of a goat, depending on what you're asking for. Voodoo, does, however, work its way into households that would be considered "more civil", in which voodoo is no differant than praying to spirits instead of "god".
There's no secret incantation to follow in "doing" voodoo, you just have to learn it's teachings and practice the religion. The best place to start is a library or bookstore (probably under "new age", despite the irony that most religions found there are pre-christian). I've found most web sources to be broad or unhelpful. Keep in mind that, like all religions, whichever source you learn from will probably be fairly unique as each household tends to have differant guidelines. Ultimatally, it all comes down to the sad truth that there isn't much magic in the world, and as much as we'd love to believe there is some obscure art out there that only a few have tapped into, voodoo would not beconsidered one of them. Bon chance.