I'm one of those super planners so here is my advise.
Make a spread sheet. I'm not that computer savy so I do a handwritten one.
I plug in (in pencil) the things we want to do.
Start with the research. I like The DIS (of course!) Baubaums book and
www.wdwig.com.
Write down all the things/parks/restaurants that interest you. Any special "events" you want to do like Richard Petty, fireworks cruises or the like. Pencil those in. Ask you fiance and her daughter to look at any guide book you buy and let you know what interests them. Then pencil them into your spread sheet. Make a sort of itinerary for yourself.
If you have anything to prebook check the day you can do this (ie. 120 days out) and mark that in red on the top of you spread sheet. Cross it off when booked. For the restaurants, think about making a PS. You can always cancel then if you don't want to use them. You can move it around, but the goal is to put in those items you really want to do and get a plan.
Keep in mind that this is a loose itinerary, not set in stone. Therefore there is no stress in doing all the things you set out. Make sure you put in break/pool time for relaxing.
Once you are ready to go, I put the daily itinerary on an individual page (you can give out copies if you want to

) and refer to the page the night before. You can then add in things you didn't do, don't want to etc.
I think a loose itinerary keeps you on track, maximizes what you do and reduces the stress.
I hope this helps. But the best advise is relax, don't try to do everything and have a great time!