Welcome to the forum.
You'll see that some girls are listed as "Hotel Reservation". These ones you MUST call to book (a time slot). A 'mamasan' or 'papasan' will answer and direct you to a hotel and room number. Beware that the 'locations' listed may not always be the same - e.g., a lot of thegirls listed in Wan Chai and CWB actually operate via hotels in Fortress Hill and North Point.
For the 'walk up' girls, they will have an address listed, and sometimes a phone number. If there is no phone number listed, you just walk up to the address and to their door - these places and doors are pretty obvious. They're very 'neon' lit etc and glammed/jazzed up so they're noticeable once you walk up the flights of stairs. If they have a 'Welcome' or 'Please ring bell' type sign, ring the bell/knock on the door and enjoy. If it says 'please wait', it means they're busy with a customer or just busy in general. If they have no sign, it means they may have stepped out.
If they have a phone number listed AND an address listed, you can try your luck just walking up but chances are they'll be busy. Best to call in and make an appointment.
For the 'phone appointments', pretty self explanatory I would imagine.
And when you've had your fun, don't forget to come back home and write up a report/review on it to share