Back in the 50s and 60s, my grandmother would set aside the feathers plucked from a chicken (after dunking in boiling water) and spread them on a paper sack to dry.
Then when she had time and a few batches, she would wash them in a tub of soapy water hot but still cool enough to put your hands into, swishing them around and rubbing them between her fingers.
Then she'd do the same in a rinse of clear hot water with a little bit of blueing mixed in. She then drained them as much as possible and laid them out on an oilcloth tablecloth to dry. I don't know if it's true, but she said the blueing helped with the smell and made them softer.
When completely dry, she'd put them into a pillowcase and hang it on the line, with about 5-6 clothespins holding the flapped over top on and beat it with a rug beater to soften them up.
Then she'd sew up a ticking, leaving a hole just big enough to stuff with her hand, sewing the last hole up by whipstitch after stuffing it full. That was how they did whole mattresses and pillows both. Lots of work, but I sure did like sinking down into those feather mattresses in the winter!