Single Sign on between Non Drupal and Drupal Site

I have two sites on two different domains. One is a drupal 6.10 site and other is a non drupal plain php site.

I want to have single sign on between these two sites. The single sign on is going to be from the Non Drupal to the Drupal site.

I have looked at Shared Single sign on and other topics posted by people but didn't get any clear ideas on how I can achieve my goal. Has someone done anything similar before? If yes then could they please post the steps they took to achieve this.

If some one knows of any module that will be helpful to me then do point me in that direction.

Thanks