Hi Torstein. I always thought that the fasteners were fitted to the factory tonneau...thats a new one for me!
Personally, I would work from the rear middle, outwards and then from the middle front fastener on the scuttle top, leaving the side poppers on the door and just behind the door until last. There needs to be some tension in the tonneau to keep it taut if you are driving with half tonneau over passenger side. Mine needed about 10mm to stretch it tight but then it may vary from car to car being hand built!
2009 Roadster 3.0 "NERO"