After you get your bags you'll walk through a hallway lined with car rental kiosks. At the end of that line of kiosks there will be a RED and WHITE kisok that says ADO. ADO is the local coach bus line that runs from Cancun Aiport to Playa del Carmen and Tulum. They're nice airconditioned busses, with a bathroom at the back and the best and cheapest way to get to Tulum.
These tickets range from $4-22 CAD depending on the date and time. Buy a ticket for the next bus to ADO Tulum Centro. Once you exit the building, walk to the right and look for a big red bus/bus loading bay (pictures 1 and 2). As soon as it arrives or you see a staff member in a red ADO shirt, ask them if the bus is for Tulum Centro.
Most of the time the bus driver will exit the bus, and start yelling the destination when they're loading the bus, bus it's best to ask to confirm. Next, show them your ticket and grab your seat. The ride from Cancun Airport is approximately 2hrs, there is usually 1 or 2 stops on the way at a resort or the Tulum Archeological Zone. DO NOT exit the bus at the Archelogical zone, you want to get off at Tulum Centro.
Photos 3&4 are photos of the ADO station in Tulum Centro, once you exit. Hop in a taxi to Selina Tulum Hotel Zone - expect to pay 400-600 pesos.
When booking your bus back to the airport at the end of the trip use Busbud.com for your ADO tickets, Tulum Centro to Cancun Airport Terminal 3. The reason you don't want to use Busbud when you arrive is that you never know how long customs will take and you may miss your bus, on the way back however you won't have that issue.