How to setup child domain under win2003

I have a single test domain running in its own W2K3 forest with funct
level 2003,
with AD integrated DNS setup on it (bird.local). I'm adding another
domain as a child (cat).

Should I first joined a new W2K3 box into the root domain, then ran
DCPROMO
to make a domain controller for a new child domain.

Thanks for help

Boris