Sew crotch seam to pin and back stitch.
Cut a fly placket and fly facing. Placket is on the left and facing is on the right. To make placket cut fabric 4 inches x the length of the zipper. Fold the fly placket lengthwise and serge the edges as shown. Cut the fly facing 2 inches x the length of the zipper. Round the bottom and serge edges as shown.
Place the zipper on the fly placket and sew on the tape closest to the serged edge.
Fold the extension on the left pant leg under 1/2 inch. This allows the zipper to sit back so it is not exposed when completed. Place the folded extension on top of the zipper and placket. Top stitch all pieces together. Stitch all the way to the end of the zipper
Fold the placket and zipper out of the way of the right hand seam.
Place the fly facing on the right pant side (right sides together) matching the seam edge and sew 1/2 inch seam allowance to pin and back tack.
Turn fly facing to inside of pant.
On the wrong side of fabric fold the fly placket out of the way and pin to secure while sewing the zipper to the fly facing. You will not be sewing the zipper to the pant front until the next step. You can press the fly facing to the inside at this step.
Sew the zipper to the fly facing only.
Match the center fronts and pin to secure. Keep the fly placket pinned out of the way.
Sew 1 inch from center front to about 1 inch before end of zipper.
Unpin fly placket and place it over zipper. Sew a curved seam from seam opening to previous seam, overlapping stitches.
Finished fly front.