Couch buttons occasionally fall off, either due to stress or normal wear and tear. These buttons are sometimes purely decorative, but in most cases, they hold the tufting in place. Replacing the button and keeping the tufting in place requires an upholstery needle. These curved needles allow you to sew through the cushion, even if you can't access the back of the fabric. Most fabric stores carry both upholstery needles and heavy-duty upholstery thread.
Cut off the button if it is still attached but loose or dangling. Remove any loose threads from the cushion.
Thread a curved upholstery needle with heavy upholstery thread. Knot one end of the thread, leaving a 3-inch tail of thread behind the knot.
Pass the needle through the couch cushion, starting at the location where the button was originally attached. Push the needle in until it scrapes the back of the cushion for attached cushions. For unattached cushions, push the needle until it reaches the back fabric, but don't allow it to pass through the fabric to the other side of the cushion.
Bring the needle back through the front of the cushion, bringing it out next to the initial needle entry point. The curve of the needle allows you to do this even if you can't access the back of the cushion. Pull the thread so the knot sits snugly against the cushion.
Thread the button onto the needle. Stitch one or two more times through the cushion to hold the button in place.
Tie the thread and the 3-inch tail together in a double knot. Trim the thread ends, and tuck the knot behind the button.