The first thing I did was cut down toothpicks to various lengths. I thought about pulling out some wire to make some fine detail brushes, but that didn't happen today. The next step was to color the brush handles. I used a Sharpie, because it was quick and easy. I didn't want to spend a lot of time making some props that would likely not get used all that often. These could also be painted, you could start with colored toothpicks, there are numerous options, including leaving them plain wood. But I chose coloring mine with Sharpie. I would recommend sealing them with a bit of finish, nail polish, anything really. I didn't do this and I ended up with Sharpie smeared all over my fingers at the end of things.
The next thing I did was pull out my bag of doll hair. This might be what stops a lot of people from being able to make these simple little paint brushes. Pretty much everyone has toothpicks. Not everyone has doll hair hanging around. If you had even a Breyer doll hanging around you could likely snip off some hair to use as bristles. I used viscose doll hair. I took out a small bit and cut one end very straight. Then I added a bit of glue to the cut end (the photo didn't come out) and laid it out on my finger and then rolled the end of the brush through it, applying a bit of pressure to get the hairs to stick to the brush. After that I cut a very narrow strip of aluminum foil, added some glue to the shiny side, laid it across my finger and rolled the brush handle through it in the same way I had applied the hair. This created the ferrule. I snipped the foil and made sure it was nice and tight, trimmed the hair so it looked more or less like a paintbrush (though none of them looked like high quality artist's brushes) and I was all set to set up Little Elecktra in some painting scenes.
I got out my paper plate making materials, made a plate, added some paints, and I set Little Elecktra up to work on her Bangles. It was fun and amused me. I have a feeling I am going to be spending too much time playing with dolls during NaMoPaiMo.
I know this was not really a tutorial on how to make miniature brushes, but hopefully it was enough to help you if you decide to make some. If I left something out or something doesn't make sense, feel free to ask me questions. I hope your dolls have as much fun painting as the rest of us!