Try using a denim needle especially designed for topstitching. This needle has a longer, sharper point that will penetrate the densely woven yarns of denim, and a larger eye for using heavier sewing threads. These features alleviate thread fraying and shredding by making a larger hole in the fabric for the thread to pass through. The fly front zipper mainly used on menswear has become a popular fashion detail on girls' and women's garments, too. On menswear the fly front laps left over right.
Instructions given here are for a finished zipper lapping left over right.
The instructions easily can be adapted for right over left lap. With right sides together, double stitch the center front seam from dot A to dot B. Machine baste the rest of the seam. Clip the center front seam to stitching line. Press basted seam open. Place the closed zipper face down on the right fly extension with the left edge of tape along the center front seam.