On RCI....You can prepay them before your trip through your TA or do it at the customer service desk when you get on board. We did it once we were on board at the service desk...Very easy.
Either way, on the afternoon of your last full day, they'll put vouchers under your door. They are about the size of a check each and state clearly on them which person they go to (head waiter, room steward etc.). You just put them in the envelopes provided and hand them to each person. You can put extra cash in them if you like (which is a nice touch).
Don't worry, they make it very easy and clear about the tipping process once you are on board.