Cut out two pieces of fusible fabric interfacing; this will be used to cover both sides of the zipper flaps. Make sure that the interfacing extends from the center front of the zipper to the edge of the flap. Use an iron to fuse the interfacing to the underside of the flaps of the zipper.
Using a 5/8 inch seam allowance and starting at the halfway point into the curve of the crotch, sew the two fronts of the pant together down to where the base of the zipper is situated. At the base of the zipper, use a regular stitch and a backstitch. Continue along the rest of the zipper with a basting stitch.
Using your shears, snip into the seam allowance, up to the stitching line at the bottom of the curve. Press open the flap seam.
Position the pants so that the right sides are facing each other and the left side of the flap of the zipper is sitting flat. Then, with the zipper upside down, lay the zipper along the front center seam. The bottom of the zipper should be sitting at the bottom of the curve.
Secure the zipper in place with pins. Stitch the zipper into place near the teeth of the zipper. Flip the zipper over so that the front of the zipper is exposed. Next, sew the zipper again, 1/8 of an inch from the fold. Take your time, as this stitch will be visible when the zipper opened.
Flip the front of the pants so that the wrong side of the zipper is facing you and both pant fronts are positioned toward each other. Position the zipper so that it’s flat and allow the zipper to fall on its own, then pin the unstitched side of the zipper. Stitch the zipper to the flat.
Turn the pants right side out. The zipper should lie flat on the front of the pants.
Mark out the location of the zipper base on the material and mark out the final stitching line; it should be 1 ¼ inch from the center of the front seam.
Using the marked line as a guide, sew the final stitching line.