The butler will help you with any aspects of your cruise, organize you dinner reservations, being able to get off the ship first, they unpack your luggage if you want them to, etc. etc. etc. ... all the little goodies that you want them to do.
they report to the Concierge, who will also look after you very well, so anything you need you can also talk to the concierge and she will organize it for you.
it's a very cool thing...really... just enjoy the ride...
the tip is really up to you. I assume you are a US citizen so you are probably more familiar and have a feel for it than me as a European has.
talk to your concierge, ask her what an appropriate amount is, without being cheap or going overboard!
Smooth Sailing Slouman