Exchange Server OWA configuration for Internet

Okay I cant figure this out. It should be easy but for some reason it’s not (for me).

I am attempting to make my sbs 2003 exchange server (OWA) accesible to users from the internet. I get a logon screen when I go to the address but any credentials I supply don’t get authenticated.

What I would like is step-by-step to configure EXCHANGE, ROUTER, and external DNS to complete this project. Any help is greatly appreciated.