24h visa how to proceed?

As long as you are flying Country A > Beijing > Country C then you would handle at the airport.

Taken from another site:
When you get off the airplane please follow the sign “Exit, Baggage Claim”. Please keep walking until you see “24/144-hour International Transfer (Apply for Leaving Airport)”. DO NOT stop on the way, such as: “Exit & Entry Certificate Service Office” and “Transit for international Flights within 24 hours”. You might never be allowed to come out.