Sand it down as smooth and thin as you want. Sometimes I will take a sharp knife and trim the sides a little when the quill is to wide still. After that you finally have a feather ready to use.
This is what a stripped feather looks like on an arrow. These have been shot for two years or more. If you look closely you will see that the stripped feathers will begin to get wavy, they still shot good. This is left wing 5" feather.
These are 4" right wing feathers that have years of use also. But these are the ones that were split, chopped, and sanded leaving the white interior for strength. No waves in these, your choice.
If you are seeing red in the stripped feather it's because on the other side I take a red felt tipped pen and color the white parts, your looking at the back side above where it bled through. The last picture has that also but it is turned away from you.
The way you sanded those is how I do it, except I sand the feather before I chop it. They sure come out nice, look just like the ones you buy. Now I am going to try some goose feathers I got from a friend, heard they do real well in wet weather. Anyone out there have any tips for the goose? I plan on just doing it like the turkey for now.