If you arrive at the International Airport [non-charter BUT sometimes charters fly in here 🙂 ] then exit the front of the airport and walk to your right. There is usually a bus there but if not either ask a porter or just walk over to the bus stand which is right at the end of the front of the terminal.
Cancun Charter Airport
Just like the International Airport, International flights do sometimes fly into the Charter Airport. The Charter Airport has a shuttle service right out the front exit/entrance of the airport. You can also take a shuttle between the airports if you want to take the bus from the International Airport instead.
Playa del Carmen Bus Station
You catch the bus to Cancun Airport at the bus station on 5th Avenue across from MacDonalds.
You do not have to pre-purchase a bus ticket but show up at least 15 minutes before the scheduled departure time.