you could affix them with a glue or fixative, that's really the only way to get them to stay "Straight" in the opposite direction of which it's been woven to run. I wouldn't use any sort of styling product or heat to work with this as any heat will cause it to become an afro puff and any styling stuff doesn't get absorbed and can leak residue onto other costume pieces, Try something like watered down white glue and when you have it all put on comb the hair the direction you desire, this will help to also remove excess glue and allow for it to dry less clumpy.
Hope this helps
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Steven Universe- Rose Quartz Gown version
Evangelion- Asuka Langley Soryu Yellow Sunddress
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