it is very simple

1. Go to some meadow or forest
2. Light a fire (small fire, not wildfire…  :D )
3. Sing and dance around it.
4. You just became a priest  :D

I all ready do this. But when i watch on videos celebration on Kupala day and so on I could see that they have some young boys and some ceremony araound them. When ceremony is finished they god some belt and feast begins. Because of this i always thouught that is some way to become a priest.