http://disneyworld.disney.go.com/restaurants
1. Find the restaurant you want and click "Book a Reservation." (or, click on the name of a restaurant you want to read more about--there will be another "Book Online" link on its page)
2. If you're staying on-property, above the calendars will be a link to associate your hotel reservation with dining reservations (if you're at your 90 days from arrival, associating it should allow you to book for your whole stay ... aka 90+10).
3. Search for a day or days, and time (it will search 1 hour before and 1 hour after whatever time you put in).
4. If it finds something you like, click "Add to Cart." If not, go back and search again.
5. Enter the captcha (the words it asks you to type in to prove you're a real person) to add it to your cart. It will stay in your cart for 2 hours as long as you keep using the site (if you stop using the site for 20 minutes, you have to start over).
6. When you're done adding all the restaurants you want, click "check out." If any of your reservations have a credit card hold or require pre-payment, you put in your CC number then.
7. YOu should get an email that has all the confirmation numbers.
I do a lot of Internet shopping, and I think the WDW dining site is one of the easiest I've ever seen, except that the captchas can be hard to read sometimes (you can click to ask for a different one if you can't read it).
You can also try it out if you want ... just pretend-book a reservation that would be within the 90 day window, and don't check out.