In the early days (including Kardec's days), séances under the auspices of one medium were common, but Kardec soon realised that they were too tempting for charlatans and advised the current frame to be adopted.
The leader is typically asserted to be a medium and he or she may go into a trance that theoretically allows the spirits to communicate through his or her body, conveying messages to the other participants. Other modes of communication may also be attempted, including psychography or automatic writing, numbered raps, levitation of the table or of spirit trumpets, apports, or even smell.
An experienced medium should be part of the group to understand the symbols and messages given by Spirits. If a medium is not present, one person should lead the group.
Evocation of spirits, especially those of recently-deceased people, was frequent in the beginning, but nowadays most Spiritists regard this practice as bothersome both to spirit, who is most likely still suffering, to his family, who may share some of the anguish experienced by him, and to the medium, who will probably face a much worse conflict and power loss.
Another problem with evocation is that there is no safe way to tell whether the spirit communicating is who he purports to be. Not all spirits can come to talk when called. Some simply chose not to come. Some have returned to source. Some spirits are not human but can be animals or evil entities. It is the reason why most medium instead remain receptive to any spirit willing to communicate and will then incorporate those who wish to speak to the living attendants. Evocation may be sometimes done, in special meetings, but only when a medium "feels" that a given spirit may be willing to communicate.
"Incorporation" is done for charitable reasons. Such reasons include bringing relief to the family of a recently deceased person, sending away some evil influence that is lurking about someone and, quite usually, helping spirits of people that died an unfortunate or unexpected death and are unaware of their state. Often such meetings include the presence of suffering spirits, blind with pain and full of wrath, rebel spirits that do not want to heed the "rules", or spirits that seek vengeance against those they feel did them wrong. In all cases, the approach is to listen to the spirits' complaints, pray for them, try to instruct them and invite them to come around frequently to share the benefit of friendship.
A mediunic meeting is usually held in a windowless room around a square or rectangular wooden table (round tables are not widely used anymore, but may be found in some places) and consist mostly of evoking or incorporating spirits. Some people (the workers) take seats around the table, with the president at one end. The rest sit on stools or benches close to the walls, usually to merely watch and listen.
The meeting is carried on with dim light so that spirits eventually willing to manifest in visible form will not find it too hard (bright light apparently makes materialisation more difficult, as the spirit requires much more of energy to become visible against it).