There's all kinds of ways to have the experience. There are sometimes ceremonies which cater to tourists, which I don't particularly recommend. In the worst case ,your interest may instigate a ceremony that wouldn't have happened without you; that can cause trouble and also be very expensive. The best way to begin is at large public ceremonies, like those at Souvenance (near Gonaives), Plaine-du-Nord (near Cap Haitien), and Saut d'Eau (in the interior); these are seasonal, so it helps to know when they are. A good many foreigners attend, so it's not so difficult to enter the outer circles. An experienced companion is good to have, someone who can stop you from crossing the invisible boundaries. If your intentions are honorable, you are almost certain to have a happy experience in these situations.