Well I'm sure there are tons of ways to approach this, but I'll tell you what I do. I start out by making a grid on a piece of paper with a box for each day of my trip. Then I list the park hours and parade and show times for each park. If they haven't been released yet, I come up with the best guess based on the previous year. You're mostly trying to figure out what night Spectro and Wishes will be because Fantasmic and Illuminations will be every night. You'll also want to note which park has EE.
The next thing I do is pencil in any specific plans I've already made, like
MNSSHP or the dinner shows since these will have to be worked around.
The next thing I do is schedule some time at each park. I always start my first day at MK, so I pencil that in. If it happens to be an EE day I may rethink that. (I avoid EE days). I kind of work out from there scheduling some time at each park, generally going with the day after EE plan. I also look at what park I want to end up at. If it looks like Spectro and Wishes will only be on one night of my trip, I'll want to end up there. I generally park hop to two parks so that would be my PM park that day.
It really becomes a puzzle, juggling what the best days are to do a park and how much time you want to spend there. I try to make sure I have 2 full days at Epcot and MK each (albeit broken up into halves) and 3/4 of a day at AK and a full day at MGM. Then if my trip is longer, I continue to add on in sort of the same ratio. I usually keep a day at the end that I'll play by ear based on what I want to re-do or didn't get done.
You definitely need to have an idea of what is most important to your group so that you know about how much time to schedule at each park. That will help you plan your days.
Have fun!!