have 4 or 5 different machine brands and some of them were so old that shirring elastic hadn't been invented, so I devised this method for old machines and no attachments. Set you machine to zig zag at about 2.5 length and 1.8 width.  The needle should swing just either side of the elastic. Tie a knot at the starting end and zig zag into position using your marked line as a guide.  Leave a tail of about 5".  Do this for each line. DO NOT GATHER AS YOU SEW. Make sure the beginning of each piece of elastic is secure by sewing a line of straight stitch down the edge. Pull up your elastic like gathering thread to the size required and secure the ends. If you pull the fabric top and bottom you will find that the elastic will align itself. Of course if you have a machine that does shirring just use the recommended method.