A separate slip to sign, just as you would for a tip on a party of five or fewer on the DDP. Instead of writing in the tip amount as you would if five or fewer, the slip will have a pre-filled-in amount computed 18% on the current menu prices of your order.
If your party is in two rooms the bill will be split but you are still seated as a party of six (whether or not you have separate ADR's) and the 18% will apply to the individual bills.
If your on the DDP, then the 18% is based off of what your meal would have cost. If you are using multiple plans (6 people, 3 couples, 3 plans), they will break the "bill" down however needed and put 18% gratuity on each persons meal separately.