Rituals are incredibly powerful procedures used to create semi-permanent spells.  The spell that is cast as part of the ritual must have a target other than self.  To perform the ritual, the leader of the ritual must know the spell used in the ritual or be able to use a Tome of Magic that allows him to cast it.  The ritual requires one participant for each spell level of the ritual (with a minimum of three).  The other participants of the ritual must have the spell on their normal spell list; have the minimum ability score necessary to cast the spell and the Ritual feat.  Completing the ritual requires one full day of preparation for each spell level of the ritual.  The actual ritual takes one hour per spell level of the ritual to cast.

In addition to any normal material or focuses uses in the ritual, each participant must voluntarily sacrifice spells slots equal to the level of spell cast.  For example, if a 4th level spell is cast as part of the ritual, all four of the participants must sacrifice a third level spell slot and a first level spell slots, or two second level spell slots, etc.  These slots cannot be used so long as the ritual is active.  If any of the participants of the ritual are killed, or chose to no longer honor the ritual, the spell ends and all participants regain the use of the spell slots.

When the ritual is finished and the spell is cast, the spell effects the subject as normal (as the ritual leaders caster level and ability modifier), however the duration of the spell increases to permanent.  If the spell is dispelled, or otherwise ends (such as a Stoneskin spell reaching its maximum number of hitpoints absorbed) it is automatically renewed after one hour so long as the subject is alive.  If the ritual was cast on an item, the spell is renewed so long as the item is unbroken.

Rituals are ALWAYS subject to GM approval.  Some spells are simply not compatible with this system.