How do I set up Outlook Web Access?

All,

Please help. I have a customer running Windows Server 2008 SP1 and Exchnage 2007. I am trying to set up OWA. I have set the external URL to https://mail.xxxxx.com/owa and there is an external DNS A record pointing mail.xxxxx.com to the correct IP. It doesn’t work. Is there anything else I need to do?
It does work internally with the internal URL.

Excuse my ignorance, I’ve never doen this before and I can’t seem to find a step by step guide on the web.

Please help.

Chubbmaster.