Go to the Pelican Parts site, find the Tech Articles section, and follow the instructions for bleeding the coolant system under Coolant Replacement.
http://www.pelicanparts.com/techarticles/Boxster_Tech/29-WATER-Flush/29-WATER-Flush.htm
Or better yet, buy Wayne's book, "101 Projects for Your Boxster" and you'll have 90% of the answers for almost any job right at your fingertips.