In King of Sartar, there is mention of a "Foundation Ritual", that is used to form a clan. Part of founding a clan is gaining a wyter. In more than one example, a living person has provided a specific otherworld being to be the wyter and, after that person's death, the wyter is 'proven' to be part of their soul and the dead person takes over as wyter. Wilmkirk and Jonstown are the two examples that I (possibly mis) remember.
For communities smaller than a clan, I would imagine a cut-down version of the Foundation ritual is used. Similarly, when a community decides it needs to replace its wyter (eg. conversion from balance to war and back), then I guess a similar ReFoundation Ritual is used.