Thread your needle with 12 to 18 inches of thread, depending on the size of the repair. If the rip is the entire length of one cushion, you will obviously need much more thread. Make sure that the thread matches the color of the couch as closely as possible. Tie a knot into the end of the thread.
Insert the needle into the inside of one end of the rip, coming out the bottom of the rip approximately one-half to one inch from the end. This will allow the knotted end to be inside the fabric and not show once the repair is finished. Beginning the repair before the actual rip also adds stability.
Begin sewing straight stitches half to one inch below the rip by inserting the needle into the fabric to one side of where the fabric begins to rip and coming out on the other side along the other edge. Continue sewing straight stitches, one beside the other until you get up to the place where the rip begins.
Sew the rip by inserting the needle into the left side of the rip and coming up through the fabric. Pull the thread taunt and insert the needle up through the right side of the rip, pulling taunt again. Sewing the fabric in this manner will produce a stronger seam than if you did straight stitches all the way up. This would produce a seam that resembles a zipper.
Sew straight stitches again for half to one inch beyond the rip, once you finish repairing the rip itself. This will secure the repair and prevent the ends from fraying or tearing further.
Tie a knot at the end of the seam and trim the thread as close to the knot as possible.