Pre-paid can only be done when you make the reservation, or by having your agent modify the reservation. You can also go to the guest relations desk on your cruise and have the tips added to your shipboard account. In both of these cases, vouchers for the tips will be delivered to your cabin on the last day of the cruise. The vouchers list the employee job title and are for the suggested tip amount for the length of the cruise. You then put the vouchers in the provided envelopes and present them to the staff on that last evening. You can also add extra cash to the envelopes if you think someone is deserving. The crew redeems the vouchers for their money.
The old-fashioned way is still practiced by Royal Caribbean. This is where envelopes for each tipped employee are delivered to your cabin on the last day. You then put cash in each and present them.
Personally, I've been going with having the tips added to my account and then adding extra cash to the envelopes along with the vouchers. This frees me from having to gather up a lot of cash on the last day.
You can see the suggested tip amounts here