Lay the spin flat and fleck (between 12 -18 inches) from foot to head. Then roll/fold (between 9-12 inches) from luff to leach and stuff into your storage bag. This is very similar to how new spins are folded from the factory...except the roll/fold is much tighter.
Just an addition, flake the spinnaker with the retrieval patches up. Makes rigging it much easier. Attach the tack line, unroll it on the hull, attach your halyard, the sheets, and then the retrieval line. Then just snuff it. If you're like me, when I first got a spinnaker boat I always had to hoist it on the beach to make sure everything was correct, you'll eventually reach the point where you're confident in everything you're doing and you don't have to flog and beat up the spinnaker on the beach for no reason.