I made the costume by creating a base first, which was actually a high waisted skirt that zipped closed, and a top that zipped at the side under the arm, in white. Once I had that fitted to me - since i didn't want just a sac hanging, I wanted to have curves, I ripped strips of cotton and tea-stained them by laying them flat in the bathtub and painting the stain on them from the top edge down, so that it would appear more 'shaded' as I layered the strips. I pinned the strips from the bottom of the dress up, then sewed them on- layering them so the layers above hid the stitching underneath. It was very slow to do, and to get the top layered nicely I had to play around a lot before I was happy. The hood is a separate piece entirely since the strips had to realistically go in a different direction or it wouldn't have sat nicely at all.
Use larger strips for the bottom of the dress and thinner strips for your middle and it will help accentuate your waist as well.
Good luck on your costume!
deviantART muro drawing

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~I'm that Canadian girl with the camera as big as her head.