We are implementing SSO and are having trouble understanding what we need to do to include our existing customer portal users.

We are using Salesforce as both our Identity Provider and Service Provider.

On page 44 of the current 'Single Sign-On Implementation Guide' under 'Enable Single Sign-On for Portals' it says "In addition to the SAML sign-on information that must be gathered and shared with your identity provider, you must supply your information provider with the Organization ID and the Portal ID. In the SAML assertion that is sent from your identity provider, the portal_id and organization_id must be added as attributes."

Could somebody please explain how to edit the SAML assertion so the attributes for the Org ID and the Portal ID can be included.  

I'm going crazy reading documentation and I think I'm only getting more confused so any help will be very gratefully received!