Visa on arrival or do I need to do something before I board the plane in departure city

You will transit in Beijing or other airports?
If you transit in Beijing, you need to do nothing before departure. If from other countries, you need to tell the airline that you want to enjoy this policy when you board the flight. Upon reaching the airport, you just need to show your passport to the officer and fill a simple form (only some basic information are required to fill) to get a stay permit at the 72-hour counter in the airport.