take note of the addressses on this website e.g. click on Tsim Tsa Tsiu, on the left frame is a list of buildings in TST, click each one and you will see some buildings have more girls advertising than others, this is a good sign, take noye of the building in the photos of the girls and use
http://www.ypmap.com/en/viewer.asp?mapService=LocationMap to print out a map if you have to
yes you just walk into the buildings, they may have a security guard downstairs but if you dont act suspicious or do anything to draw attention they will never question you, then its just a matter of going up each floor and knocking on doors until you find one you like. The doors will have a sign saying 'welcome' or come back later, only knock if the sign says welcome also dnt knock if there is no sign, there are private residences in these flats.
my method is to go through this site make a list of girls I want to see and then go through the list when I get to the location e.g. I write down
champagne court
Floor 1 - A1 C1 D2 E3
Floor 2 - C1 C2 D3
Floor 3 - D2 A1 C5
... etc