For example, use 20cm nylon coil zipper. Mark the needed length on the zipper. (This time, 16cm.) Trim off the excess zipper. Fabric (outer and inner) length is 19cm. Sew the outer fabric and zipper with right side together. Take a look closely. Sewing line is here. Specifically, sew 5mm from the edge. Sew the outer and inner with right side together. Take a look closely. Sewing line is here. Fold the fabrics, and iron. Then stitch the edge. Sew the other side of zipper, do the same way. It’s done.