how to go to sandhu hotel from the Hong Kong Airport?

You can take bus no.A21 at Airport (Ground Transportation Centre), get off at Cameron Road, Nathan Road. The duration is about 65 mins. It costs 33 HKD per person. Then walk southwest around 180 yards (150 meters) to arrive at the hotel. And the return way is the same. :-)

instead of cameron road can i get off at haiphong road instead? what would be the route from haiphong road to sandhu hotel?

Yes, you can. After hopping off at Haiphong Road, Nathan Road stop, you can see the Harilela Mansion in front of you. Your hotel is right on the mansion.

what is the landmark that i need to get off ? that i am already at the haiphong road.

Do you see the Harilela Mansion? If yes, the hotel is just beside it. :-)