Hey there! First off, thank god 8chan is back.
Doll hair cannot be cared for like human hair. Unlike human hair, dolls rarely need brushing or any sort of maintenence, however play often causes wigs to frizz out or other issues occur.
This post is probably going to be retardedly long because I get questions about my dolls wigs all the time about IE: curls. I can make up a how-to-do image if people are really curious, but I can link a few places.
First things first, the doll hair brush may or may not be useful. Its probably absolutely useless on curly hair. You are good with the toothbrush, but often I thin out the hairs on the toothbrush in order to 'fold in' hairs that are being pains in the butts sort of like folding in say, dough when making pie.
for hairs that get 'crimped' (ie, at a 45 degree angle, or more extreme, or are sticking up from the center part) often cannot be saved, especially if they are broken. I gently trim these few hairs off with a pair of nail scissors.
Synthetic wigs (especially wigs made from Kanekalon) are susceptible to static. Best thing for this is 1 part fabric softener to 3 parts DISTILLED water. If you use tap water and your water is hard, it can build up deposits and water spots on your dolls hair, and it will be a pain in the ass to get off. You should NEVER spray any product directly onto your dolls face. It will be a pain in the ass to get off, and dolls that are not sealed properly - well, I take off the wig beforehand for extreme styling, and then use a towel under the doll for quick touch ups. Think about the consideration your barber takes for your shoulders and face when you get a haircut, same thing.
NEVER USE OILS ON SYNTHETIC DOLL HAIR. IT WILL DAMAGE THE STRANDS.
A ton of hair products for humans contain oils, especially products that tout 'moisturizing' consequently, often cheaper products (ones that dont have oils and are all synthetics, filled with silicones (dimethicone) are often best for wigs.
For BJD ideally you should never be in the position to have to 'wash' the wig, but in the event of extremely damaged wigs (extreme frizz, think the barbie your little cousin has) one brand i like is Suave - their conditioner is nearly solid silicone and alcohols (which get washed out) and their soap is generic SLS.
This is a (sorta) good show as to how to re-curl loose, long curls:
(check out 10:03 to 10:40)
https://www.youtube.com/watch?v=_fxIGlWjQvUFor extreme, tight curls:
Doll hair that says "heat-resisting fiber" means usually a somewhat higher grade of synthetic hairs that can handle high-heat situations. They are referring to 'tight curl' or other extreme styling which you can use a low-heat blow-drier and near-boiling water in order to achieve "boil perm" styles (scroll down to "Re-Curling" same method applies. [Again, use only DISTILLED water.])
http://www.mlppreservationproject.com/hair.htmlYou don't have to use curlers either. You can use drinking straws for really dense, thin curls, or even Boba Tea straws for wider, thick 'sausage' curls in the tighter styles that Leekeworld often achieves.
When in doubt, don't use a product that you don't know about, and err on the side of caution.
I can take pictures if you really want and maybe do a tutorial, but up to you OP. Once you get the hang of it, you can try more extreme curling and other styles.