Apply fray check to the tear. Spread it on and around it to prevent the tear from growing.
Thread a thick thread the same color as the couch through a needle.
Pull the two sides of the tear together and sew tightly together from the bottom up. If it doesn't feel secure, sew it again from the top down. If the tear is too large or you can't pinch the sides together, use a piece of fabric that matches the couch's color and sew it on top of the tear.
Knot the thread securely at the end and cut away the excess thread with scissors.