You can have your tips automatically added to your onboard account. There will be a form delivered to your cabin early in the cruise where you indicate if you want the tips added to your account or not.
If you do My Time Dining then you need to pre-pay your gratuities before sailing. You will not need to do anything on the last night.
If you have your gratuities added to your account. On one of the last nights you will receive little "cards" and envelopes. You just stick the cards in the envelope and you're done. You can add more $$ is you desire.
I personally would like to do away with the cards completely.
There is no need to bring envelopes.