I haven't used the one pictured above, but I have used a couple of similar services in Tokyo and Osaka. It was pretty straight forward.
You go to the website, choose the guy, email them in English and they will response. If you want to visit them, they will tell you a location to wait (usually a 7 Eleven or gas station close to a metro station) and the boy will come and pick you up. The site will depict how much is for each type of service (they call it course) and what the boy will do (top, bottom or massage only). Kind of like going to a restaurant you choose the number of courses and then the protein:-)
They typically have a predefined course of action when you meet up. Confirm the time, shower with you, massage, loosen you up, and penetrate. They communicate with phone translation app. Comparing to what I experienced in Bangkok, I am actually pretty happy with the service I received in Japan, but of course the price is a bit more than double.
I am actually surprised that there is not a subforum for Japan and Korea.