I believe her outfit (and Chihiro's in the bathhouse) is a kind of outfit known as a jinbei.
I'm having trouble finding tutorials for ones that aren't for children online, so you'll have to do some hunting.
Currently working on:
Benny Gecko: Fallout: New Vegas
Momoko Koigakubo: Ghost Stories (Dub)
Snow White -- Snow White and the Seven Dwarfs
Princess Tiana -- Princess and the Frog
Malory Archer: Archer
Kamala Khan: Ms. Marvel
Anna Cornonation Dress: Frozen
Vader Eloha/The Queen: OFF
Princess Suwa: Mascot of the city of Suwa, Japan