How to make a paintbrush?

Answer 1

Find a horse or a camel.
Pull a whole bunch of hair out of its tail (or mane for a horse). Take care not to be bitten or kicked.
Tie the hair around a thin stick., using a separate cord.
Soak the tied area with glue for permanency
Trim to length and desired shape.

You can substitute your own hair for the animal hair above. & maybe cut it instead of pulling it out. (You could cut it from the horse or camel, too.

Horsehair and camelhair are reputed to make the finest brushes. Human hair is probably finer, but doesn't get a lot of billing.