One important thing to remember with ISA/TMG web publishing is there is usually no "one size fits all" article or guide. Most guides you find can be very help but often provide step by step instructions for very specific scenarios. Many of the configuration steps are dependent on your particular security requirements.
In the article you reference, most of the information discusses configuring the OWA sever itself...configuring the cert and exporting it to the ISA server. Also, this article specifically discusses an SSL to SSL bridging scenario. Is this a requirement for you...or would an SSL to HTTP bridging scenario suffice? Also, you then need to decide about Certificate authorities. Will you require a Public Certificate Authority, so the external clients are not nagged about Trusting a certificate, etc....
This article also discusses the use of a HOSTS file entry, which is not required for TMG. Strickly from a TMG perspective, first make sure the Internal Network object contains all IP ranges. Next, use the wizard to create the Sharepoint publish rule. Many of the answers should be clear. Also, the following article may help get you started:
http://technet.microsoft.com/en-us/library/cc984488.aspx
Regards,
Richard Barker (MSFT)
