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.
My username is an incredibly stupid pun that only I will get.
Currently working on:
Momoko Koigakubo -- Ghost Stories (Dub)
Scarlett O'Hara Drapery Dress -- Gone With the Wind
Deandra -- MPGiS
Anna Cornonation Dress -- Frozen
Vader Eloha/The Queen -- OFF
Princess Suwa -- Mascot of the city of Suwa, Japan
My favorite cosplays:
Brittnay Matthews -- MPGiS
Mom Lalonde -- Homestuck
Princess Kenny -- South Park