No tip is necessary. You must call ahead and select a lady and make an appointment. This is unlike a spa where you arrive and then make the arrangements. Normally an incall place will want a number to call you back at so they can check out your bona fides before making the appointment.
When you get there, you meet the lady and spend some time with her.
Big "don't" - Don't be too specific about what activities you expect over the phone. There's a certain amount of decorum and discretion expected of you. By all means discuss the physical type or rough age of the lady you would like to see.
Friends has a website listed under "Kasey's Kittens", although I understand it's out of date. If you arrive and unfortunately do not like the girl you've arranged to see, you'll probably be expected to pay a cancellation fee unless another more acceptable young lady is currently available to see you.
It goes without saying that you should be polite and reasonable when dealing with the people involved.