Much easier to get 'AID v4.01' and allow it to do it for you! But if you keen on doing it yourself, the XBMC wiki tells you how to do it! http://www.xboxmediacenter.com/wiki/index.php?title=As_a_Dashboard
personally i would stick to one of the standard dashboards like UnleashX, EvoX, or Avalaunch. XBMC is an ok dash but i know many people who have had trouble when using it.
when i was using auto installer i did something wrong and I dont have a dashboard or get to the softmod menu but i can still play games how do i fix it