For those included in the prepaid gratuities, it's your stateroom host, and then in the main dining rooms it's the head server, server, and assistant server. How much to tip them extra is really up to you and how well you feel they treated out. The amount will also vary-we never interact with the head server very much (not because they're bad, it's just the nature of their role), so we'll leave them a small increased tip, but not as much as, say the server and assistant server, who we really interacted with a lot and they took great care of us. As PP said, you can do that in cash, or go to guest services and have them add the additional amounts to your onboard accounts.
Any other person who you could tip, you can do so at the point of sale (Palo/Remy servers, bartenders, etc.). This doesn't have to be cash either, you can add it just like you would when paying with a credit card.