Yup, its just as easy, either way. I'll explain it from the top.
Lets pretend its a family of 3, and a $100 bill. The server drops off the check. You are on the DDP, and its mid Feb, so don't even worry about how grat was included on the 07 plan. The bill will be for $100, and tell you that 18% is $18, and 20% is $20. There will be a blank to write in a tip, and then a blank for the total at the bottom. If tipping in cash, just write cash. If plastic, write in whatever tip you see fit. We will use $20 for this example. Write the tip for $20, and the total for $120 then. Put your room card in the black book. If putting the tip on the room(and you obviously have room charging abilities), you're done. If putting the tip on any other form for plastic, put that plastic in the book as well and give the black book to the server. When the server swipes your DDP card, it will ask the server what the guest tipped. The server types in $20. The meals are now gone, but there is a $20 balance remaining on the check. The server then swipes whatever form of plastic the tip is going on. Lots of paperwork will spit out, a couple autographs from the guest, and the guest will be on their way.