It's been more than a year since I did it, but I think there were some posts online that helped, and some pictures too. I did find it much easier to install the shoulder bolts after removing the carpeted piece where the seat belts retract into. Then, if I remember correctly, I got the bolts in with the top up. A strong flashlight helps, and a long ball-end allen t-wrench with the threaded part of the bolt stuck through a plastic baggie so it can be held on the wrench while starting the threads.
This might help:
Installing Spinlocks - 986 Boxster Top Issues and Solutions - RennTech.org Forums
Or this:
Boxster Board Message Museum
Once you try these tips, it's really not hard. Let us know how it goes. Good luck!