Regardless of if you prepay, add a credit card on file, or pay your onboard account balance in cash, on the next to the last night your stateroom host will leave a sheet of paper with slips that you tear off for the server, assistant server, head server and stateroom host with the standard amount. You also receive envelopes that you can place those in for each person (and you can include additional cash if you would like) that you then hand to your server. This isn't required as they will get the tip regardless but it's good etiquette. If you had particularly bad service you can deduct from the normal tip at guest services.