The tip or "Gratuity" will automatically be added to your onboard account. It will be $10 per person/per day. You can go to the Customer Service desk and have the tips removed from your account and tip in person, at the end of the cruise.
I suggest leaving it on your shipboard account. It makes things much more simple for you and the staff. If you wish to lower your tips, because of poor service, you can do this at the customer service desk. If you wish to give someone extra, you can do this at the end of the cruise. You can enclose it in an envelope with a personal note. An additional amount is neither expected or required.