«If seche-cheveux is hair dryer in french. How do you make it plural?»
It ends with a "x", so it's already plural (cheveu: singular, cheveux: plural). When it's a verb(sèche or drying)+name(cheveux or hair), it doesn't change wheter or not it is singular or plural.
Scoob94P: Just just wrote horse dryer
Here's a link (in french) that explain how to conjugate composed name:
Pluriel des noms compos