Depending on how close you are, you may be able to just tell her upfront, but we internet strangers don't know your friend like you do.

Otherwise, assuming you're both single, it's best to slowly lead into it (asking to touch or play with her hair when you meet in person, complimenting it,stuff like that). Then once she's used to the idea of you talking about and messing with her hair, you can tell her.

If she's in a relationship though, there's a lot less room for error. You could still do the stuff above but you'd need to tread more carefully so there aren't any misunderstandings.

Moreover, if you're already in a committed relationship yourself, I believe you're better off pursuing your fetish with your partner and nobody else. Others may say differently but that's my take on it.