You simply write in the amount you wish to spend, and give the card to the server like a credit card. Just be sure to be clear that the gratuity should go on the card, and not on your room key (if you have charging set up).
That, and make sure you are tracking the balance so that the transaction will go through... (btdt )
We often use the rewards cards for this, and on other trips my travelmates and I have put in cash up front and bought a gift card onsite, which we use to tip all week - an easy way to split tipping costs without tracking who charged or paid what all week.