You make your reservations exactly 30 days ahead of time with Disney Dining, and pick up your tickets at Town Hall (or at your hotel if you are staying onsite and Vacation Planning made the reservation for you.) Then, you line up at 8:00 to get your assigned seat. Some families line up really really early for this...the Dobies lined up at 7:30 last summer and got good seats. After you get your assigned seat, you can leave and have fun, and return a half hour or so before your show to get your dessert box and beverage. If you made reservations for the first show, you can stay in your seats for the fireworks.
The desserts are yummy, and more than you can eat in one sitting! There are also fruit and cheese in the box. One of the dining or fantasmic threads has pictures.