I assume that you mean you want to create a custom portal which when used by your customers displays your portal, and not the default portal. If so, here is what I would do:
1) Make a copy of jboss-portal.sar
2) Edit jboss-portal.sar/conf/data/default-object.xml to contain your portal's layout (keep the portal name as 'default', though)
3) Delete jboss-portal.sar/portal-samples.war/WEB-INF/default-object.xml (this file defines the Test page, which you will not want)
4) If your portal contains custom portlets, places their war files under jboss-portal.sar (this is required only if you reference the portlets in the default-object.xml from step 2)
5) Create your own custom theme (so you portal announces itself as your portal and not the JBoss Portal) The Portal docs contains a detailed chapter on designing a theme, or you could make a copy of one of the existing themes at jboss-portal.sar/portal-core.war/themes and then use GIMP or Photoshop to edit the existing images.
6) Starting with a clean database and JBoss server, deploy the updated jboss-portal.sar.
I hope I didn't forget any steps, but I think that the above is all you need to know.