Fill a bathtub with water and 1/2 cup mild liquid soap.
Place the duck feather cushion in the tub. Press down with your hands to completely submerge the cushion.
Allow the cushion to sit for 15 minutes. Use your hands to press down and circulate the soapy water through the cushion every five minutes while you wait.
Drain the tub and refill it with water. Press the cushion with your hands again to force out lingering soap from the inside.
Drain the tub and refill it one final time. Allow the cushion to sit submerged in the water while you press on it to continue rinsing it out.
Remove the cushion from the water once all soap has been rinsed and squeeze excess moisture from it. Allow the cushion to air dry for 24 to 48 hours or until it is completely dry.
Use your hands to pull up on the dry cushion to re-fluff it before returning it to the sofa frame.