My window opened last night. Some of the things I did to prep for it were:
Go to the fp site and play with the calendar/selections to be able to click with ease. It also familiarizes you with where the attractions are in the list you're going after.
I made a list, by order of importance/popularity. For instance, my first day at the parks is May 4, but I went for May 5th first because that particular day had harder to get fps than other days. Next, I went to Sunday, May 8 because that day had the next most popular. Then I went for the rest. My vacation starts in the middle of a week, and my first choice (Thursday, 5th) at midnight already had a couple of fps no longer available (A&E, MSEP). Move to a later date in your vacation to get harder to get fps. Saturday and Sunday (7th and 8th) had everything available.
Another thing I did to prepare was to check the wait times to determine the fps I really needed. I also checked the 30 day window to see what was readily available (most of the time) at 30 days vs what was almost always unavailable. I basically went in, grabbed what I wanted, then went back in and modified the times before moving on to the easier to get fps.
I made my list in alphabetical order. I also had the dining times listed so I could work around them It just made it easier to make decisions. I also listed times for any shows we want to see, to make sure I didn't do something like get Toy Story Mania in the only time frame we have to see Indiana Jones. Some shows have limited show times.
I also was able to go back in and modify fps to get another for my granddaughters. On Wednesday, we are going on Pirates, Splash and Space Mts. The girls are not tall enough for Splash and Space so I modified the times first, then changed it altogether to the Magic Carpets, which we will do while the grownups are riding Splash.
Those are the sort of thing which will help you make decisions and the process will go smoothly. While some fps can be modified very easily much later, there are a few that, once a few days have gone by, become very difficult to change.