They only braid on the Magic right now as far as I know (but having never sailed the Wonder, I can't guarantee that). Don't know if that will change in the future.
With that being said, as of 5/11 here's how it worked.
You go to the Goofy Pool Sunday morning at 9:00 and they are 1) braiding on a first come first served basis, and 2) taking appointments for later in the day/week. We booked for the first sea day at 4:00 (first available other than during Key West time) and we were in line by 9:05. Take your kids navigators with you when you book, because I mistakenly booked it during the Flubber making time! DD said she wanted her hair braided more than she wanted to make Flubber, but she really wanted to do both! Bad mommy!
Also, it helps if you know exactely what you want when you book. The options are 1) Headband style, beads at the top 2) Headband style, beads at the bottom, 3) normal braids. They only have certain people who can do the headband style, so those appointments seemed to be more limited.