Each of the two piece frames has an outer perimeter and cross pieces. They just sit (tightly) on top of the cant rails. You hold each piece towards the middle, bow the centre rod downwards and then slide each piece forward or backwards as applicable. When you release the bow, spring tension holds it all in place.

Although, I have seen cars with a small Phillips head screw though the rear corners of the front piece into the cant rail.

Cheers, Ian