There is a long section of netting, and a short section. Lift the end of the mattress, and slide the long section underneath as far as it will go, so that the short section can then be folded up at 90 degrees to form the net. (At this point the large section of net is hidden under the mattress, with just the smaller section visible). You can then attach the four plastic clips to the four small rings in the roof.
You will end up with a small vertical net at the head of the mattress, designed to catch pillows & small children! It shouldn't cover the hatch in any way. Do note that it is best to climb over the net if you can, rather than just sitting on it, as you can break the roof clips. They are cheap enough from VW if you do manage to break them.
I always check that the net is folding nicely out of the way when closing the roof, as it will prevent the roof from folding completely flush if it bunches up.
Simon