Like Dave said, there will be forms once you get onboard to have prepaid gratuities charged to your SeaPass card. Your SeaPass card is your room key, and it will also be used for anything you purchase onboard. You can either establish your SeaPass account with a credit card, or cash. Credit card is the easiest.
Usually for prepaid gratuities, your stateroom attendant will leave a form in your room that you need to fill out and return to the guest relations desk if you want prepaid gratuities charged to your SeaPass card. You need to fill it out and return it to a box at Guest Relations when specified, which I think is by either the third or fourth day of the cruise.
On the last day of the cruise, your Stateroom Attendant will give you envelopes, and if you prepaid gratuities, vouchers to place in the envelopes. Place the appropriate vouchers in the envelopes, along with any additional amounts you wish to give. Then give them to each of your dining room staff the final evening at dinner. The Stateroom Attendant one I usually just leave in the room in a conspicuous place for the attendant to see.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)