At the airport ground transportation center, you could take the bus A21 and then get off at 14th station called Middle Road, Nathan Road. Then, please walk across the Nanthan Road to the west part of the Middle Road, where the Kowloon Hotel is located. The last bus will leave from Hong Kong Airport at 24:00 and take you HKD33.
Ps: please pick up a free map once arrive at the airport, which will lead you the way clear after getting off the bus.

Hello. I would like to check that the advice is still the same now. Do I find the Shuttle Bus Service on the ground floor of the international airport (arriving from Shanghai). Is it still A21 to get to the Kowloon Hotel? I am arriving at 12:25 pm. Thank you.

Yes, you are right about the bus location. Don' worry! The signs or marks are very clear there to help to find the buses at the airport.

You are arriving at 12:25 after noon, right? As for Kowloon Station, you can surely take bus no. A21 and get off at Middle Road, Nathan Road and walk to your hotel.