The same thing happened to me. Download these instructions and follow steps 8, 12, and 16. I had to experiment with the tension so the top does not rub against the clamshell edge and squeak while driving. When that happens it's enough to drive you insane.
http://images.wikia.com/boxster/images/e/eb/DIY_Boxster_Conv_Top_Canvas_R_R.pdf