1. Add DNS entry for new Web application (i.e., http://portal-ext)
2. Add IP address if not using host headers explicitly
3. From Central Administration -> Application Management, select 'Create or Extend a Web Applicaton'
4. Select 'Extend an existing Web Application'
5. Select the Web Application to extend in the drop down selector
6. Always use 'Create a new IIS web site'
7. Enter a description (doesn't matter, but it should make sense to you)
8. Enter a port, and it should be a standard port 99% of the time, like 80 or 443
9. Always use a host header, even if assigning ip addresses. You'll thank me for this when you scale WFE Servers :-) And, don't forget to add the second AAM if accessing your new Zone via http://portal-ext and http://portal-ext.mycompany.com
10. Path doesn't matter, but defaults work well
11. Choose NTLM unless you currently use Kerberos
12. Don't allow anonymous, you can do it later
13. Decide if you will use SSL (only changes default internal URL)
14. Remove :80 if you want. It isn't needed for standard ports
15. Select the Zone - Zones matter. When you apply policies, you are asked for the Zone, not the URL. Write this down! For example, http://portal-ext is on the 'Extranet' zone for http://portal
Now, you can change authentication, anonymous access, create custom policies, modifiy in IIS.
Cheers,
Ben Curry, CISSP, SharePoint Server MVP
Mindsharp
http://mindsharpblogs.com/ben
http://www.microsoft.com/mspress/books/10623.aspx