Hold the loose button on the back of the couch and pull it upward, along with its thread. Snip the loose thread with scissors carefully, without tearing the couch fabric. Set this button and the thread attached to it aside. Pull out stray threads that remain on the sofa with tweezers.
Cut an 18 or 20-inch length of upholstery thread and thread it into an upholstery needle. Align the ends of the thread before forming a knot. The double-strand thread now measures 9 to 10 inches.
Form a 1/8-inch stitch at the spot on the back of the couch where you will attach the button. This stitch secures the thread to the couch back.
Pull the upholstery needle and thread through the shank at the back of the button. Extend the stitch in the back of the couch, through the fabric and batting, before taking it out again. The curve of the upholstery needle will grasp some batting behind the couch fabric.
Sew the thread through the back of the couch and then through the button’s shank, until 3 to 4 inches of thread remains. Stop stitching when the thread sticks out through the couch fabric, but not through the shank.
Wind the thread around the shank three to four times to strengthen it and keep the button from coming loose. Pull the thread and snip it off. The tip of the thread should be buried in the couch's interior.