Here is a step by step photo tutorial for the front zipper instructions as they appear in the Cargo shorts instructions from step 21-28. You can buy the Cargo Short pattern here, and the Chino pants steps 14-21. You can buy the Chino pants pattern here If you want to practice the technique, come join Wardrobe By Me pattern support and find the pattern pieces for the zipper under files. Use a 5" / 12 cm zipper.
Step 21:Transfer the circle mark at the bottom of the front fly to the fabric. Place the front pieces RSF and sew them together at the front crotch to the circle. Baste from the circle-mark to the waist
Step 22: Fold the fly pieces open and press.Turn the piece right side up, and topstitch from waist to crotch 1/8” / 3mm on the right side of the fold.
Step 23: Switch to zipper foot.
Flip to the wrong side and fold the front pieces to the left, and place the zipper face down, with left side of zipper tape against the fold. The zipper head should be 2” / 5 cm below the waistline. (For the chino pants the zipper is placed 3/4" / 2 cm below the waistline).
Pin and sew the right side of zipper tape to the fly. The bottom of the zipper should be aligned with the circle mark.
Step 24: Flip the zipper right side up and under stitch the zipper tape and seam allowance to the fly facing.
Step 25: Flip the front pieces to the right, then pin and sew the other side of the zipper tape to the fly.Switch back to regular presser foot.
Step 26 Flip the pieces to the front, and mark the bottom of the zipper, so you know not to sew through that point!. Draw the shape of the fly with chalk 1 ¼” / 3,5 cm from center front.Sew the fly facing to left front.
Step 27 Fold the fly extension (15) right sides together and sew the top and bottom together. Turn right side out and press.
Step 28 Sew the fly extension to the right side of the fly facing. Place it 3/8” / 1 cm below the waistline.
Turn the pieces to the right side and sew a bartack at the bottom of the fly.
Unpick the front edge basting seam. You now have a functional front fly zipper.
Add bartags as illustrated.