Things You'll Need
Utility knife or electric carving knife
When your sofa cushions get saggy or lumpy, but the fabric and frame are still in good shape, replacement cushions can give your sofa new life. You'll have to determine the size of foam you need to fill each cushion, then purchase high-density foam designed for use in furniture cushions. If your sofa's back cushions have zippers, you can easily replace the foam in those, too.
Remove your cushions from the sofa and unzip them. Turn the fabric inside out if possible; this makes it easier to remove the cushion. If your cushions are attached to the sofa frame, such as back cushions, carefully pull out the stuffing.
Peel away any batting or fabric that's wrapped around the foam cushions. Set it aside. You'll use it later to wrap your replacement foam pieces.
Measure each foam cushion's length, width and thickness. Add 1 inch to each measurement so the foam will fill the cushion cover properly. If your cushion isn't square or rectangular -- T-shaped, for instance -- measure the longest and widest parts of the cushion. Purchase a piece of high-density cushion foam that's large enough to cut down to size for the number and shape of cushions you have.
Draw a pattern for each foam piece needed. Lay your new foam on a hard work surface. Place your old foam cushions on top of the new foam. Draw around each cushion, onto the new foam, with a marking pen. Make each outline 1 inch bigger than the cushion all around.
Cut out your foam pieces with a utility knife or electric carving knife. Be careful not to mar your work surface underneath.
Wrap each new foam piece with the batting or fabric you removed from the old foam. Insert each wrapped piece into its appropriate cushion cover. Zip up the cushions and put them back on the sofa.