Carnival does not give you anything to give to the waitstaff to show you paid the tips but they do give, if I'm correct, a list to the waitstaff so they know how much they have coming, etc. I usually on the last night, shake hands with the waitstaff, tell them the tips were added to my acct. and if I want to add something extra at that time I slip it to them in the hand shake and say something like " and here's something extra for all your hard work ", etc.
On RCCL, you can have the option of doing it the old way with cash or go to the pursers desk, get a form to fill out and they will add it to your on board acct. Then the last evening the cabin steward will leave envelopes on your bed with the different names, like cabin steward, waiter, asst. waiter, head waiter, etc. There will be vouchers for each, you detach the proper voucher and insert into the proper envelope and then hand them out after dinner, etc. But, as I said, that's on RCCL.
I like the fact you can add the tips to your acct as it saves bringing more cash to keep up with and worry about and is just easier, at least to me.