Surround the tear area with seam sealant to prevent further ripping of the sofa. Adding sealant also strengthens the repair thread's ability to close the tear.
Thread a needle with upholstery thread that matches the color of the sofa. Tie a knot at the end of the thick thread to prevent it from slipping out of the needle.
Insert the needle about 1/2 inch from the tear and bring the needle to the other side by pulling it all the way through. Continue to sew the tear closed by bringing the needle back and forth through the tear.
Tie a knot when finished to ensure the thread doesn't come out in the future. Trim the excess thread with a pair of scissors.