Publish Sharepoint 2010 using tmg when internal and external link is the same

Hi all,

My internal SharePoint site is http://sp.xxx.com.sa:13500 , I want to expose it to internet using TMG with the same link so that users use the same link from inside and outside. We are using windows authentication in SharePoint IIS. Kindly how to do it step by step on TMG and keep in mind that we do not want users to be asked for credentials more than one time when accessing the published external link from internet and when working inside it as well.