Sorry I don’t have any pictures for this like I do for my Leia hair breakdowns; I’m kinda doing this one on the fly!
So here’s my best reference for the Mustafar hairstyle; this is from padawan’s guide:
And here is an other screencap.
To be perfectly honest, my Mustafar costume was a quick, very non-serious costume I did just to match my husband’s Vader, so I didn’t try too hard with the hair; looking at it now I’m thinking of better ways I could have done it, but I’m going to share how I did it in the past when I wore this costume.
This is probably the best picture I have of my own hair in the costume:
I used three hairpieces for this: a clip-on ponytail (braided) and 2 of my ANH Leia wefted hairpieces. Here’s a guide of what is what. The parts outlined in red are the loose wefted pieces – the part in yellow is the clip-on braid.
I put my hair into two low pigtails in the back, with the clip on ponytail right in between them. Here’s a really awful illustration I just made with my mouse.
For the two pigtails of my own hair, I rolled them up EXACTLY like I do for my ANH Leia buns, then add one of the wefted extensions on top. It’s basically a mini-Leia bun, so you can check out that link for some more info.
Once both of those are rolled up, the two buns + the clip-on braid should all look like one big piece, or close enough.
Here’s a shot of me wearing it where I had the side buns braided instead of loose, so you can pretty easily tell which part is which:
Hope that helps!