Hi Melissa!
On my first trip to WDW, I tried to plan almost every little detail (I am one of those type-A planner-type people!). However, I have learned that at least for us, there is such a thing as TOO much planning, at least in terms of scheduling. Our first trip, we scheduled a lot of PS's, and we ended up cancelling a few because we decided we wanted to do something else and couldn't get there, or we ended up not feeling like eating whatever it was we were scheduled for.
What I've done for my upcoming October trip (2 days at Universal, 5 at Disney), I've just decided which park(s) I'm going to on which days, and scheduled only the priority seatings for the restaurants I REALLY want to go to and those associated with an event like
MNSSHP (you could book at LTT before the 90 day window with a ticket), and the Fantasmic! package. I've only scheduled dinner PS's this trip, we're going to wing it for lunch, especially since we plan to split parks. That way, if we feel like it, we can even go to one of the resorts for lunch -- which don't seem to be very crowded.
For the Disney parks, for some reason I find that splitting a day into 2 parks works better for us and we're not so tired, so on my itinerary, I just have "AM-MK, PM-EPCOT," etc.
Because you're going during peak season, you might need to plan a little more, and it depends on the size of your party too. If there's only 2 of you, it'll be easier to get into restaurants, esp. if you go early or later than others. We even got a group of 7 into 50's Prime Time in MGM in the middle of July without a PS, but we probably had to wait about 15-20 minutes.
Because you're there for 12 days, I would also schedule 1-2 "free days" where you can just get up when you get up, and lounge around or go shopping or something different. It'll help keep you refreshed the rest of your stay.
So I would say at least have a general idea of what park you want to do on what day, the "don't miss" attractions, and PS for the places you really want to eat (or if you like to eat at the same time everyone else does). And, make sure everyone in your group understands that your itnerary is just a guideline and won't mind being flexible and going off the original plan.