Materials: large feather, old ballpoint, scissors, super glue (or tape)
Cut off the top of the feather. You will find it is hollow.
Then take out the ink tube from an old ballpoint (but still with ink inside) and cut off a big part of the length. Because the hollow part of the feather will get narrower at the end, you won’t be able to fit the entire ballpoint tube inside.
Apply super glue inside the feather. Place the ink tube inside and let it dry. Instead of glue you could probably also just wrap some tape around the feather at the top. I prefer the glue method though, because it leaves less visible marks.
When properly dried, you can write with your feather as if it was a regular ballpoint.