Go to the Disney dining site:
http://disneyworld.disney.go.com/dining. Choose the restaurant that you're interested in and click "book a reservation." On the next page page, click where it says to enter your reservation information, then enter the information requested . Once you do that, click on the calendar next to the date that is showing (should be the current day's date), and move the calendar forward to the dates of your stay. The dates that you can book are in blue. If tomorrow is day 90 for you, the dates for the next ten days from your arrival date should be in blue. Choose your preferrred time to dine and the number in your party. Click on the date you want from the calendar, and a window will come up indicating the openings that are available for the date and time that you chose. It looks an hour each way from the time you chose, so if there's nothing for that time, maybe check an earlier or later time.