You just go to any of the suspected massage shops and book a half hour or hour massage as normal and pay for it. You go into the room, get naked and lay face down. The girl will come back into the room and massage your back. It is likely she will begin to play with your willy and/or ballsack through your legs. You may become aroused. She will then ask you to turn over.
You most likely will have a hard or semi-hard willy at this point and she will obviously notice. It is common at this point for her to point/touch your willy and ask if you want extra. You will then just negotiate what you want and what you'll pay and then she will do it, you pay her directly before you leave the room.
Done.