Dennis,
The replaceable brush I showed in the last series of posts is a commercially made item available from Gesswein - it is a "bristle end brush". They come in various sizes and are horse hair mounted in a brass ferrule. I thread the end of the shank to make them easy to replace. As a consequence, I don't have to do any bundling or gluing. However, I have made several other brush types that did require making up a custom brush. If using horse hair, or something similar, the easiest way I am aware of is to bundle a sufficient number of strands and then seize the bundle (with thread, dental floss, or artificial sinew) at intervals determined by how long you want the brush part plus the length of the seized part that will end up in the handle. I cut the individual brushes free with cable cutters (the blades surround the hair bundle and cut cleanly) just behind the seizing (leave a 1/16 inch so the seizing doesn't roll off the end) and put a drop of glue on the end of the seizing and the bristles. The individual brushes can then be crimped or glued into whatever handle you come up with. In like manner, I have made a few brushes where the seizing covered about 2 inches of the bundle. One end I cut about a 1/16 inch from the seizing. The other is cut about 3/4 inch away. I then covered the seized area with a braid of very fine leather thongs. The whole brush is leather and hair - no solid handle, but the leather braid cover makes the handle part very stiff....cute variation on the theme.
Dave C