Remove all of the cushions from the sofa. Unzip the cushions if they are zipped, or split open the seams with a seam ripper. Hold the sharp edge of the seam ripper at the corner stitch and start ripping away the seams.
Insert the cut foam into the cushion with your hands. Zip the cushion cover closed. If you ripped the seams with the seam ripper, make a 1/2-inch fold of the edges of the upholstery fabric and place small pins an inch apart all along the edge.
Sew the edges with an upholstery needle and small stitches to secure the sofa seat. Complete this process for all of the cushions.