Here is information from the official website of HKIA:
If you have an onward boarding pass, please:
Follow the directional sign to one of the Transfer Areas;
Go through security screening;
Check your gate number and time, and reach your boarding gate at least 30 minutes before departures time.

For passengers without an onward boarding pass, please:
Check your airline’s designated Transfer Area;
Follow the directional sign to the designated Transfer Areas for check-in;
Go through security screening;
Check your gate number and time, and reach your boarding gate at least 30 minutes before departures time.

For more details, you can go and check the information on the website.