i am looking at a flight to myanmar with a transfer in guangzhou (leaving next day coming and going)

I see that guangzhou as of 6/2013 allows for the 72-hour free visa if I book the flight over the internet how do I apply for the 72-hour exemption? I may stay at the airport and not go into the city do I still register with the authorities? the lay-overs are 15 and 22 hours so I feel I might just stay at the airport.