Horse Hair like banjo vellum has to be wetted before it can be worked.
One end is tied, In these days with thin wire and then set in a ball of resin. Originally probably melted Rosin, These days probably Araldite.
Once it is secure the fixed end is fitted to the far end of the bow. The hair (wetted) is combed or brushed so that the individual hairs run parallel. Once again they are tied off and fitted into the FROG on the bow. The skill of a professional lies in getting everything to lie straight, flat and even.
The finer points of fitting are avalable in at least one book - which was where I read up about 18 years back before making a simpler bow for a Psaltery.
Quack!
GtD.