Repair an open upholstery seam with upholstery thread and a special needle called an upholstery needle. An upholstery needle is a heavyweight, curved needle that allows you to sew from the outside of the seam. Both the needles and the thread are sold in fabric stores. You can also find the needles in packages of home repair needles in the notions section of a discount store.
Cut a yard of thread and thread the upholstery needle. Knot the ends of the thread together.
Reach inside the open seam and bring the needle out through the seam allowance on the inside of the cushion. Bring it out in the crease of the fold on one side of the opening 1/4 inch from the end.
Push the needle into the fold on the opposite side of the open seam. If there is cording in the seam, run the needle through the cording as well. As you sew, keep the thread perpendicular to the seam by always inserting your needle directly across from the place where you brought it out.
Bring your needle back out through the crease in the fold on the same side of the opening. Continue sewing this way until you reach the end of the opening. Pull the thread tight.
Sew the seam again in the opposite direction if the stitches are too large. The size of the needle will determine the size of the stitches. Try to bring your needle out halfway between the first set of stitches. Continue until the stitches are 1/4 inch apart. Pull the thread tight and knot it.