(Nope, I'm gonna write one more post before starting that paper. . . . Don't look at me like that.)
If your girlfriend has admitted to having a shampoo fetish - as in, she loves having her hair washed - don't be alarmed, I have advice for you!
1) Have shampoo and lots of conditioner at your place, stuff that she can use. They can be "girly" products (like Herbal Essences), or products that are gender neutral (Pantene). Really, you should keep stuff a girl can use somewhere in your bathroom at all times, because a girl needs to be very low maintenance to even consider using a shampoo made for guys. You may want to anticipate using lots of shampoo, and buy something inexpensive like VO5 or Tresseme. Maybe throw some shaving cream or shave gel into your arsenal while you're at it!
2) Know that being shampooed in the shower is great, but girls with this fetish usually want to be shampooed over the sink, over the tub, sitting up, in the bath, etc. Ask her where and in what position she'd like to be shampooed.
3) As far as sex goes, don't expect to receive sexual gratification during the shampoo, or even after. Don't be one of those assholes who feels entitled to sex or a blowjob just because you do something nice for a girl. Ugh, you don't want to get me started on this . . .
4) If you're unsure how to actually wash her hair, say "hello!" to YouTube! Watch videos on how to wash hair, watch videos on channels from LatherFantasies, ShampooGuys, TheHairfun, etc., they'll show you how it's done!
5) Don't be afraid of being too rough! Go with your instincts, and if you're too rough she'll tell you to be gentle. Just to be safe, encourage her to correct her if you're too rough or if she wants you to be more vigorous.
6) Focus on the scalp! This is the biggest mistake men tend to make when they wash my hair, and I'm guessing they're not the only ones. The point is to scrub and message the scalp, so only work lather through the ends if you're getting ready to pile it all on her head. Of course, if you do that, be careful! Piling can lead to tangles, and tangles lead to hair damage when it's time to comb the hair out afterwards.
7) When you're not shampooing her hair, she'll also appreciate it if you massage her scalp, brush her hair, or even gently stroke her hair. Ask her if she likes having her hair pulled, it might be a nice move to try in the bedroom.
Okay, NOW I'm gonna write my paper!