Hi, I will arrive at 10:45 HKIA, and would like to go to Guangzhou by ferry.

Well, if you arrive at 10:45, the earliest ferry may depart at 15:20. Here are the specific procedures.
1. Following the signs to purchase ferry ticket and present baggage tag receipt at the Ferries to Mainland/Macao Ticketing Counter at Transfer Area E2
2. Scaning ferry ticket and check your baggage status at the ferry boarding entrance
3. Boarding the Automated People Mover to SkyPier
4. Boarding ferries to Guangzhou