PULL IT OUT OF THE GOOSE.

Quite seriously, so far as I've heard, and read in a couple of the "old time traditions" books, no preparation is required. Generally a fairly large wing feather would be used, since they're much "fatter," thicker walled, and more sturdy than feathers from the body.

A period of drying likely would be needed, since a freshly pulled feather is rather moist and a bit "limp." If one uses the "found feathers" that fall out of the birds fairly regularly in a poultry yard of any size, the complete instructions are "pick one up and play."

You likely will find that a "quill" of any kind will not stand up too well to the metal strings at tensions commonly used on today's dulcimers, which does raise the interesting question of how the strings on modern dulcimers compare to what was used in the era when the use of a quill as a plectrum may have been common. If "folk" strings were natural gut and/or sinew, one might wonder about when, and how rapidly, the transition to more modern strings happened, and how the use of the quill fits into the changes.

John