Hi, Ashleigh!
In order to book a seating at
Cinderella's Royal Table, a Guest not on the Disney Dining Plan will need to pay in full at the time of booking,
Because you are on the Disney Dining Plan, however, your process to book will be slightly different.
On the main
Disney Dining Reservations page, you'll click on the link beneath the reservation fields that reads, "Use my dining pan to book pre-paid reservations."
From there, you'll just need to enter your confirmation number and your arrival date, and the two credits per person will be deducted from your remaining allotments.
The folks at Disney Dining are always ready to help, too; give a call at (407) WDW-DINE for further assistance.
Have a great trip!
See you later, celebrator,
-Deborah B.