I wrapped Belle up in a towel and a plastic Target sack to protect her cloth body. Then I filled a spray bottle with a 50/50 mixture of water and liquid fabric softener. I bought a free & clear variety (Target's store brand, I think). Then I completely saturated her hair. Dripping wet. I slooooowly combed it out, in small sections, from the bottom up. (note: Belle did not have open/close eyes. If you're working on a doll that does, please tape some cotton balls or a washcloth over the eyes to protect them, too. Water inside of eyes like American Girls have can rust and ruin them.) Belle definitely shed some hair, but overall, this was much easier than I feared, given the mess we started with.
Important tip for doll hair: don't use your brush! Most brushes that people use have plastic bristles. Those, when used on synthetic (plastic) doll hair, cause static, frizz, and mess. Use a metal brush on doll hair. And don't use the metal brush on people hair in between doll stylings. Your hair oils aren't good for them, either. I used a metal wig brush from Sally Beauty Supply. They're cheap. Go get one for your dolls. American Girl also sells them, but they're spendier. You can also use a metal toothed pick on dolls with curly hair. That would have been a nice tool for Belle, but I didn't have one.
After a first brush out, this is what she looked like. Yikes! No more curl! Yes, we had to sacrifice some curl to eliminate the knots. I left her to dry overnight.
The next morning, she had sticky hair from the fabric softener. I rinsed it thoroughly in cool water and then brushed it out again. MUCH better. As her hair dried, a lot of the curl bounced back. I can't guarantee that this will always happen. Cristina and I had agreed, though, that a straight haired Belle was better than a ratty haired Belle, so it was worth the risk of curl loss to us. If we had wanted to put the curl back in, there are many many tutorials on how to curl doll hair. Usually it is called a "hot water perm" if you wanted to google for method.
After Belle's hair dried a second time, I styled it, dressed her, and she was ready to head back home!