Serve an apprenticeship. Find a practicing voodoo priest or priestess and ask if you can work with them. Traditional magic -- including the English version -- has always been learned through experience, rather than books. You don't read about how to grind monkey paw, you practice it.
Be prepared to do the dull tasks. A non-paying apprenticeship may take years. The priest will only teach you the skills as he thinks you are able to handle them. Ethical practitioners make sure their students don't have too much knowledge too early, as it could be dangerous.
Guard the secrets. Voodoo chants or incantations aren't something you can rattle off in the bar to impress your friends.
Learn a system of shorthand and take copious notes. The idea behind learning shorthand is that it is unlikely anyone else will be able to decipher your work. Since you don't have any textbooks to follow, make your own.
Ask for progress reports. From time to time -- say, once a month -- ask the priestess for an assessment of how you are doing. Keep track of what she says in your notebook and search for ways to improve for the next time.
Take advantage of any professional development opportunities. People involved in the dark arts visit each other so they can talk in person, rather than over the phone. Remember that at one time being a witch could mean a quick trip to the stake.
Develop an attitude of helping people. Rather than mumbo-jumbo love chants, ethical practitioners are interested in improving their client's health and situation. Also remember that bad spells -- be they voodoo, ju-ju or American Indian magic -- can bounce back if not administered properly.