Assuming that it begins the same as OU did this generation, the council will draw up a couple of potential initial banlists. Then the community (just badged users?) will be able to vote on which of the choices they would like to see in the initial banlist. In practice, this will mean that mons such as Darkrai, Genesect and Shaymin-S will probably be allowed at the beginning of XY, but mons such as Mewtwo, Kyogre, and Arceus will be banned from the start.How does a new generation of pokemon get tiered? Do they get lumped into OU, while a few gets into Ubers?
Then usage for the first few months will determine UU, and usage for that will determine RU and so on.
During this time, suspect tests will be held, which in BW started off by banning Deo-A, Darkrai, and Shaymin-S. Eventually the pace of bans will slow down and the metagame will stabilize