You call
DCL and ask to have prepaid gratuities added to your reservation. Just decline to pay for it during the call (you won't be required to have it paid for until the PIF date).
Each guest is expected to pay a certain amount of gratuities for your stateroom host, dining room server, assistant dining room server, etc. There is a set cost per day that DCL charges each guest, and they will bill your onboard account for it if you don't prepay or have the amount changed at Guest Services (it is discretionary). I don't like having to pay for gratuities at the end of my cruise, as it's just another bill. I prefer prepaying.