Never brush it when it's dry again, and don't touch it with your hands when it's drying.
My hair went from POKER straight to wavy and is now on its way to curly. I used to do all sorts of curly girl/wavy methods, with diffusing, but now I am lazier...
I get a handful of mousse, put a splodge of gel on top, mix it then run the combined products through my hair when it's soaking wet. I comb it through to make sure the product is root to ends - make sure your hair is dripping wet, can't emphasise this enough. Once my hair is smoothly brushed and coated with product, I swing my head upside down until curl clumps form, then gently scrunch it with a microfibre towel over my hands. This both forms waves/curls, and takes out excess moisture. Then I leave it to air dry, without touching it. It takes AGES to dry naturally without touching, but I only do it once a week.
Once it's dry, I scrunch out any crunch.
Sometimes I will diffuse dry it but that takes ages too. I tend to wash my hair on a night that I'm staying in, and just let it dry while I'm watching tv. For the first time in my life, I understand where the 'not tonight, I'm washing my hair' thing came from. I genuinely have to schedule washing my hair now.
BUT I don't touch it at all with a brush or anything else during the rest of the week.