Shower advice

My daughter is 7 years old and mentally delayed due to hydrocephalus. I’ve been trying to teach her for years how to shampoo and condition her hair but she still doesn’t know how to. I’m so frustrated because I have to tell her every night what to do and when to rinse, etc. her dad says he just tells her to get in the shower and wash her hair if I’m not home but doesn’t know if she actually does it. Idk if I should just do the same or what. How can I make her learn and know she’s actually washing her hair correctly ? She washes her body fine. Help me! I’m so frustrated over this situation.