iirc, you have to have atleast 8 units then right click your general, in the lower left of the unit info screen should be a "join crusade" icon. click it and you now joined the crusade. enjoy the free upkeep in provides =)
also, if you WANT to start a crusade, you need to have a high standing with the pope and you simple "request" a crusade for a particular area. if your standing with the pope is high he will approve of it if not he will err not do it.
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