how to book tickets for ferry from hong kong internatinal airport to shenzhen

It is no need to book the tickets in advance. From the HKIA, you can easily buy the tickets to SZ.
In HKIA, it is easy to make the transfer to ferry to reach your destination by following the direction boards in the airport.
At HKIA, you can buy the ticket and present baggage tag receipt at the Ferries to Mainland Ticketing Counter at Transfer Area E2, then go to scan ticket and check your baggage status at the boarding entrance. Then you can board the Automated People Mover to Skypier.