How stupid. I see now: the slit is cut all the way thru the Styrofoam and the slit is as wide as the barbs of the feat. I was visioning it as a slit that is 1/2" deep the length of the feather.
As usual, as I become obsessed with something, I did some hunting and found this site. It seems to work well but not nearly as fast as this Styrofoam method:
Jimmy Younger Method