I played with it some and learned a few things.
It takes a lot more hairs than I thought it would.
Easiest was to whip finish thread near the middle of the bundle of hair, clip the hair about 1/4" from your whip finish where the bundle starts to get really tight, then rubber cement the hair bases along with the thread. Once good and dry, clip the other side of the thread to the length you want. I found anything longer than about 1" was worthless.
Horse hair gets really soft when wet. The brush works fine till the moment it gets a little wet.
Horse hair is in general pretty limp. Shorter is better on the length you use, but it's never going to be good enough when you get it wet.