Soul gem: There's a substance called "spirit gum" (sometimes called "theatrical glue"). It's a body glue used for adhering stuff like elf ears, gems, and prosthetic makeup to the skin.
Make sure your skin is clean so you get maximum grip, and I recommend shaving off any tiny hairs you might have in the area because they're likely to pull and hurt when you remove it later. When you make your soul gem, you might want to have a knob on the back that you can nestle inside your belly button for a little more surface adhesion.
You can remove spirit gum with a remover solution, there are a bunch of brands out there. There'll be more info on this stuff in the makeup subforum.
If you wanted to wear a flesh-tone bodysuit under the costume, you could glue velcro to the back of the gem and have it velcro to the bodysuit.
Skirt: First work with a plain A-line pattern and get the angled hem looking right. Once you're satisfied, split the pattern apart vertically and add the box pleats. Fold the box pleats in place and cut the bottoms so that the angles all match up.
FF9 cosplay? Hit me up!
2014: Anime Los Angeles, Anime Conji, and whatever else falls into my lap
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