Check to see whether the sofa cushions can be turned upside down on the sofa. Though some sofa cushions are made to sit on the sofa in only one direction, others look the same on both sides and can be flipped over. If you can flip the cushion over once you repair it, you won't need to take quite as much care with the repair.
Cut away any damaged sections of the sofa, such as stained sections or fabric with cigarette burns, before attempting to repair the cushion. Use scissors to remove the marred fabric and remove as little of the fabric as possible.
Pull the fabric or leather together if the damage is a small vertical tear with no gaps between the sides. Use a standard needle and thread in a color that matches the spot on the sofa cushion that needs to be repaired to sew the fabric together by hand. Keep the stitches small and close together to minimize visibility of the repaired tear and to keep the foam securely inside the cushion.
Get a patch for the sofa if you the rip is too large to pull together. As long as you have a solid-colored fabric or leather sofa, you should be able to find a piece of fabric that matches the sofa at any fabric store. Cut the fabric down so that it is slightly larger than the tear in the cushion and either sew the patch into place or use fabric or craft glue to glue the patch over the tear.