This is interesting. I just buy them pre-knotted, but this tool would let you use all different feather types
https://www.youtube.com/watch?v=GD3Y_pf21J4
Years ago I made a cunning small pointed wire hook from a long but quite soft dressmaking pin. It is set into a short thin wooden dowel handle. Lets me tie knots quite readily. Not dissimilar in technique.
I to have a wee hook from a piece of wire set in a wooden dowel similar to Vaughans description. Before that I used a pair of tweezers. Works with rubber legs as well
Make a circle with the fibre & pull the fine end through & tighten.
Tight Lines
Davie McPhail has a video on tying legs with a Crochet hook here: https://www.youtube.com/watch?v=LHUM8FBQPdk
Barry Ord Clarke uses a knitting machine needle here: https://www.youtube.com/watch?v=EDZ6xZ6ogoQ
Those look even easier