There was a post a while back on the 'general questions' board about water only washing. The post had a link to the long hair forum. Many people over there use water only. The link had a detailed description on how to do this. I'd try searching their forum.
Here is what I think I remember reading:
I think people use oil with a soft brush to remove scalp build up, much like removing cradle cap with babies.
Strait haired people have better luck with this method since the sebum can travel down the length of the shaft easier.

I'm not very familiar with Water Only. Most people here cowash. If you don't get many replies to this post, try reposting it in the General discussion for curly hair forum. Put something like 'water only washing question' in the title, so it will get the attention of people familiar with the water only method.
My advice to you and everyone is not to get too caught up in the rules of any method. Listen to your hair. If your hair wants water only with an occasional cowash, do that. If water only works, do that. For example, I have wavy hair. If I followed the rules, I'd low poo once a week. My hair is dry and doesn't need a low poo, so I don't low poo. I'd definitely suggest consulting water only experts on the long hair forum for advice.
HTH (a blog to help new wavies, go to the how do I get started page)
2a ( 2bFi (Fia), very fine, thin, low/normal elasticity, porous, CG, doesn't get weighed down
Cowash: Tresemme naturals
Rinse out/leave in: Renpure Organics (red)
Curl enhancer: FSG
Gel: BRHG, Salon care aloe, Ecostyler Krystal (normal dews)
PT: ION effective care, IAgirl's gelatin PT
Low Poo: renpure (red), Ion swimmers