visit visa for hong kong on pakistani national from saudi arabia

First thing is to prepare the relevant documents: copy of your passport, the application form attached with a passport-style photo, hotel reservation, flight itinerary, proof of your financial support, and your travel plan.
Next thing is to post your application to HK immigration department to apply for it. Here is the postal address:
Receipt and Despatch Sub-unit
HK Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, HK