The only envelopes we've received the past year are for the Maitre'd....so on the last cruise we specifically requested extra envelopes from our cabin steward. We usually try to talk to our cabin steward throughout our visit...so we prefer to hand them the extra money (in the envelope) although I have left it on the vanity before dinner that last night as well.
After we finish our dinner that final night we say our good byes to our wait staff...it's at that time we hand them their envelopes.
We usually give the Maitre'd his when we arrive for dinner. One cruise I thought the assistant Maitre'd did a better job than the Maitre'd. I didn't feel like I could totally ignore the Maitre'd....so we gave him a minimal amount...but I also gave the assistant an envelope and expressed our gratitude for his services. He was a tad surprised.
The amount that you give in my opinion is a personal thing...and of course depends too on how much you can budget. I usually take along the highest amount we can give. I take it in differing bills so if the service wasn't as good as we hoped I can reduce the amount appropriately. If the service was as good as it should be....then I just transfer the money to the Carnival envelope and put the persons first name on it.
It will be interesting to see if there is proper "etiquette" for your question.