This thread will tell you everything you need to know and more:
http://forum.sex141.com/eforum/viewthread.php?tid=19916
To answer your specific questions, yes you just go to the walk-up and knock. You can politely excuse yourself if the girl is not to your liking and move onto the next door. Please be considerate and don't knock if the sign indicates the girl is with a customer
For hotel girls, you speak with the papasan. Their English is typically very limited.