well rules vary in most, but basic rules:
starts with a couple who are in some kind of relationship. you 'sign-up' of the congrats game by filling out a short form
(LN:
Dparent: (age)
Dparent: (age))
each round, something (either a child, pet, or significant other) is added. there may or may not be a namebank or name list to chose from, depending on the congrats host.
that's really all there is too it. you just... create a fictional family. hope i helped! =)