Doing anytime dining is like going to a restaurant; you show up and there may or may not be a line. When it's your turn, you can tell the Matre'd your preference for a table (small, medium, large, table for two, by the window, etc.). You may or may not have to wait to get your preference depending on how crowded they are and what's available.
However, you can call the dining room during the day and make a reservation, if you'd like. And, as I mentioned, if you really like a particular table and time, you can create a standing reservation for the same table and time every night.