If your couch is lumpy and and you find yourself sinking into it when you sit, you may not need to replace the whole thing. If your cushions are squashed, misshapen and flatten out when you sit on them, then it's probably the cushions -- not the couch -- that need repair. Restuffing your own sofa cushions can save you a lot of money, while making your couch seats as firm and comfortable as the day you bought it.
Locate the zipper and remove the cushion from the cover. If your cushion doesn't have a zipper, locate a seam on the side or back. Carefully cut open the stitching with a seam ripper until you can remove the cushion.
Measure the length, width and depth of the cushion. Record these measurements. Purchase a piece of medium- or high-density foam that is slightly longer, wider and the same depth as your cushion.
Lay the foam on a table. Place the old cushion on top of it. Trace the shape of the old pillow onto the foam with a marker, holding the marker vertically. Cut the foam to the same shape with a carving knife, holding the blade vertically.
Add the length measurement to the depth measurement. Double the sum to determine the length of fiberfill batting you'll need. Cut the batting to the length you calculated, and slightly wider than the cushion.
Spread the batting on the table. Lay the new cushion on top of the batting. Line up one edge of the cushion, width-wise, with the edge of the batting. The lined-up edges will be the back of the cushion. The width of the batting should slightly exceed the width of the cushion.
Lift up the cushion. Spray the bottom side of the cushion and the top of the batting where the cushion was lying with a spray adhesive. Replace the cushion, adhesive side down, on the batting, once again lining up the edges.
Spray the top of the cushion, and the edge opposite the back edge (this will be the front edge) with adhesive. Spray the rest of the length of batting that's spread out on the table as well. Wrap the batting around the cushion, pulling it taut.
Spray what will be the back edge of the cushion and wrap the remainder of the batting around it. Trim away any batting edges that overhang the sides.
Insert the new cushion into the the cushion cover. Zip up the cushion, or resew the seam with a thread the same color as the fabric.