This is one of a few methods I use to apply a fly-front zipper. I used lightweight fabric for this sample, so it is best to interface the right front fly extension (facing).
Applying the zipper
Cut the trouser panels out. Trim the left front fly extension, ensuring that it measures 1.5 cm from the centre front. Interface the right fly extension with a 4.5 cm (1 ¾ in) wide piece of interfacing. Overlock the raw edges of the crotch, side, and inside leg seams of the front trouser panels.
Pin the front crotch seams right sides together and stitch from the marked zipper opening, up to within 4 cm (1 ½ in) from the edge of the inside leg.
Fold the left front edge under, ensuring that 6 mm (¼ in) is moved beyond the centre front seamline. Press.
Fold the right front fly extension along the marked centre front and press. Place the fold of the left front next to the zipper teeth. Pin and baste in place using a zipper foot.
Align the centre front fold of the right front with the centre front notch on the left front and pin it in place. Pin the zipper tape face down to the right fly extension on the wrong side of the trouser.
Stitch the zipper tape to the fly extension while keeping the fly extension away from the trouser.
Mark the stitching line on the right side of the trouser using a chalk pen and a template or hand-basting.
Topstitch along the marking, ending with a curve. Remove the pins at the centre front.
Interface one half of the fly shield when you are using lightweight fabric.
Fold the fly shield in half, press, and overlock the raw edge.
Pin and stitch the fly shield to the left front, close to the foldline. Remove the basting.
Staystitch across the zipper tapes while the zipper is open.
Close the zipper and stitch the lower edge of the fly shield to the fly facing on the wrong side.
Stitch a bar tack across the lower seamline (if desired), catching in the fly shield. Open the zipper and cut off the excess zipper tape.
