When you're in the bathroom, doing your hair?
"Ich mache mir (gerade) die Haare" is correct.
Note that there is a difference between German and English: In German usually the dative ("mir") is used where English uses the possessive adjective ("my"). In linguistic terms, the former is called an external possessor, whereas the latter is called an internal possessor.
"Ich mache meine Haare" which exactly corresponds to the English sentence is also possible.