GF = Grand Floridian Resort.
For the slipper dessert made by the bakery at the GF, you need to call ahead by at least 48 hours. I called Narcoossee's directly and they directed me to the bakery (I don't know what that number was). You give a credit card number and get a confirmation number - be sure to get that as that's the only proof that you've ordered it. You tell them where you're dining (any table service restaurant at the GF), and your reservation time. They'll ensure that it's there for you. Upon arriving at your table service meal at any of the table service restaurants at the GF, you just tell the waiter that you have pre-ordered the slipper dessert. If you're on the dining plan, you would get a dessert through the plan in addition to the slipper dessert. The slipper dessert made by the GF bakery is $14.00. It's made of white chocolate, has berries in it, and has chocolate mousse filling. It's delicious.
While I've never had the one at CRT, it's my understanding that you don't need to pre-order this; it's a dessert option with dinner.
Hope this helps.