US Citizen JFK - CAN - SIN and Back

We are both US citizens. JFK - CAN - SIN with about three hours in CAN. Return is SIN - CAN - JFK with about 50 hours in CAN. From what I read, on the outbound, we need not apply for any visa (TWOV)? On the return, once we arrive in CAN, at the airport, we apply for the 72-hour free transit? No need to apply for any of those in advance? Do I need to notify China Southern on the return that we will be applying for the 72-hour free transit? Thank you for the assistance! Your site is very helpful. I just want to be sure that I get everything correct.