Make the actual feather. As detailed as it gets.
Get a feather, break it apart. Study it. Replicate it in 3d.
Possibly, use a scanner and take a back/front image of the feather so you have a diffuse to mimic and a way to model with measurements.
From it, make a flat 2d one like we do for leaves.
(Normally that’s around 7 verts. 5 to really cull it).
Bake the high poly to the low poly to get normals/diffuse/opacity.
Use the hair tool on the object you want feathered.
Use the feather object in the render section.
Possibly disable or add children. Depends on the animal.
Definitely look up how to set the thing up correctly so that hair length affects the object. (It’s done differently though different versions).
Brush the feathers into position.
At this point I would suggest also baking down to a low poly. At a distance you cannot really see individual feathers. And swapping out makes performance a lot better.
Last but not least. Go ask at BlenderArtists… they’ll have better answers.