I have some doubts on the stay permit for the 72-hour free transit.

I'm mexican and will be flying from los ángeles to Bangkok, my layover is 8 hours in beijing. I see o dont need a visa, but where can I get the permit that the articule talks about? Also on the way back from Bangkok to Los ángeles (after 17 days of being in tailand) ill be flying back and my layover will be 14 hours. I need another permit right?