If you are using a sewing machine, then sew two rows of basting stitches along the open top edge and the open bottom edge. Sew one row 1/4 inch away from the edge and one row 3/8 inch away from the edge.
If you are using a hand needle and thread, pull the thread through the eye of the needle and tie a thick knot on one end. Sew a large stitch around one opening.