You can go online at 6:00 am. You can book 180 days out (from the first day of your ressie, plus ten days. If you are staying for more than ten days, you have to book those in the next booking window.
You want to make your most important ressies first. Then work your way down the list. It holds everything in your cart for two hours, so you have plenty of time. I did eight TS ressies in about fifteen minutes.
The nice part about online ressies, is that if there is not a time available for what you asked for - it will suggest other times, as well as other restaurants. It suggested another place I would never have even been to, since CP at MK was already booked solid for that day. We loved it!
You do have to keep an open mind; sometimes it is the experience that is more valuable to you and the time doesn't matter - sometimes it is the other way around.
When you are finished, you want to make sure you fill in the part that asks you for an e-mail address to receive all your confirmation numbers - you cannot retrieve your ressies w/o the confirmation numbers and the phone number you booked the ressie for confirmation (usually your cell phone number).
I put my ressies, as well as the confirmation nos. in my cell phone just in case I needed them.