My version:

Reference images:

Analysis (whoo, scientific): This style is actually two styles. The first, and most-worn one, consists of two small braids wrapped around the head, leading into a large braided bun high on the back of the head. More hair hangs down in the back past waist length with several braided loops hanging from the braided bun. You can see in pictures 1 and 4 how the braids wrapped around the head are set high and kind of "folded" at the corners, giving it a more square look. Pictures 2, 5, and 6 show how the wrapped braids lead into the main bun. The bun has an odd profile look, as you can see in the same pictures. Pictures 3 and 5 show the braided loops hanging from the bun. The smaller braids hanging down appear to be rope braids. The second style is in pictures 7 -9. As you can see from 9, the hair hanging down in back is much shorter, and the hair is piled higher on the head. There are more braids, but they're less organized and are mixed in with unbraided hair. Most of the braids are rope braids instead of normal 3-strand braids. She has this style during one or two scenes, in between scenes with the other style. It's pretty obvious (well, to the hair-obsessed) that her hair changes practically mid-scene. I'm assuming it was a mistake, although the style made it into one of the promo shots for the movie (picture 7). In either style, there are no fasteners or accessories visible.

Directions: Directions for style 1: Divide your hair into two sections, with the top one being larger. Gather the top section into a half ponytail. Braid it into multiple sections. I recommend 2 for the braids around the head, two for the looped braids, and 3 or 4 for the bun. You'll probably have to experiment until you find the right amount of hair per braid, with the bun ones being larger than the wrapped braids and the wrapped braids larger than the looped braids. The two looped braids should be rope braids (see Dreamweaver Braiding for directions on rope braids) while the rest are regular 3-strand braids. Use LOTS of bobby pins to hold everything. That's it!