start with the shape of your feather. in this case it was a "s" shape.
create the shape of the feather. not all feathers are perfect so i did a gap in the middle of the feather. This is just a rough out line it wont look so hard when you start the next step.
start adding little whispy lines. You should sweep out. This gives the effect of a thinning structure.
for the details of the feather leave a millimeter line of white running up the middle. If you add to many lines threwout the feather will make it look too heavy and dark.
To finish the feather add some dark lines that are closer together and spontaneous. This part of the feather is closer to the bird and gets more ruffled up amongest the rest of the feathers. Add some darker lines and highlights and your done.