Place the sofa cushion pillow on a flat surface and examine it. If the sofa cushion pillow has a Velcro or zipper opening, open the cushion. If not, use a seam ripper to remove the stitches from a 4-inch section of a side seam.
Press the cushion as flat as possible using your hands. Make a mental note of where the cushion feels like it needs new stuffing.
Take a handful of stuffing and push it into the places within the sofa cushion pillow that need to be refilled. If the entire cushion needs to be restuffed, fill the pillow with a handful of polyester fiberfill at a time. If the sofa cushion pillow's stuffing is old or otherwise breaking down, remove all of the old stuffing and refill the cushion pillow with new filling.
Continue refilling the sofa cushion pillow until it returns to its natural shape. Take care to force stuffing into the corners with the stuffing stick if necessary.
Turn the edges of the 4-inch opening under, touching the wrong sides of the inside fabric, and pin.
Hand sew the 4-inch opening in the side seam shut or close the cushion with the Velcro or zipper.