Hi there,
You don't tip at the table each night. On the next to last night of your cruise, your room steward will leave tip envelopes in your stateroom. Each will be labeled "Server," "Asst. Server," etc...
You put your tip in that envelope either in the form of cash, or in the form of a "receipt" given to you when you charge the tips to your onboard account at Guest Services. If you choose this option, you may want to do it early in the week as the lines get long later on. You then give these envelopes to the appropriate people.
HTH,
Julie