In this section, we will be sewing a fly zipper. Sewing a fly zipper can be challenging in that there are several steps and it is important to follow notches and markings exactly so that the zipper (and waistband in later steps) will line up correctly.
Drink some water, take some deep breaths and let's get started!
Before we begin please have ready:
  • Zipper
  • Zipper shield
  • Scissors
  • Fusible tape (optional)
  • Marking pen and ruler

Double-check that all markings are transferred from your pattern to your front pant legs at the center front and the waist edge.

Finish the crotch seams and fly facing edges before you sew up the seat seam. Options include serging, zig-zagging, or binding.
Tip: If you are using a bulkier material you may want to avoid using binding for the seat seam — this is a better treatment for lighter weight pant fabrics.
Serging or zig-zagging the entire seam and then strengthening the seam using topstitching (this step is included later on in the instructions) will create a stronger and less bulky finish for these close-fitting pants.

Starting at the center back yoke, pin the two pant legs together along the crotch seam ensuring that all seams and notches match. This is quite tricky where the flat fell back yoke seam meets up and may take several tries.
Stitch until 1/2" (1.3 cm) below the zipper placement mark at the center front. Check that your seams line up nicely and reinforce this seam by stitching over it one or two more times (this is an area that needs to be very strong). Refrain from pressing this seam until you are finished sewing your fly.
Fold fly facing extensions under and press as follows (see the diagram in the instruction booklet and the image above for reference):
The Left Front (if you were wearing the pants) will act as the TOP, the most visible part of the fly. Fold and press the left front fly extension to the wrong side at the notch FURTHEST from the curved fly facing edge.
The Right Front (if you were wearing the pants) will be BEHIND the Left Front and will be folded and pressed at the notch CLOSEST to the curved fly facing edge. This will create a smaller facing and a 1/4" (0.6 cm) extension.
Apply interfacing to one of the zipper shield pieces.
With the right sides together match the two zipper shield pieces along the curved edges. Stitch along the long curve at 5/8" (1.6 cm). 
Grade and clip the curves.
Turn the zipper shield right side out and press neatly. Serge or bind the remaining long edge.
Position the zipper shield so that the curved edge is facing towards the right and the serged or bound edge is to the left.
With the zipper facing up, place the zipper on the zipper shield. Match the edge of the zipper tape with the finished edge of the zipper shield.
Using a zipper foot, stitch along the LEFT side of the zipper close to the zipper teeth.
TIP: I find it easier to install zippers using a zipper that is a longer length than stated in the materials section. Using a longer zipper makes it easier to move the zipper pull out of the way when sewing. If using a longer zipper, match the bottom of the zipper with the bottom of the zipper shield. Let the top of the zipper tape and zipper pull extend beyond the top of the zipper shield.
TIP: Instead of using pins, use fusible tape to keep the zipper in place. 
Using a zipper foot, edgestitch the zipper and zipper shield to the RIGHT FRONT PANT fly extension. Line up the pressed edge of the fly extension on top of the zipper and close to the zipper teeth. Sew to the circle zipper placement mark on the front pant. Make sure the zipper stop is lined up with the zipper placement mark, even if the zipper extends over the top of the pants. (If using a longer zipper, the zipper will extend over the top of the pants).
With the right sides facing you, set up the fly as it will look when it is finished (with 1/4" (0.6 cm) overlap). Place a pin through the right and left front at the center front to keep the pieces in this position.
Turn the pants around so you are looking at the wrong side of the RIGHT FRONT PANT. Lay the zipper flat (you will be looking at the underside of it) with the curved fly facing and zipper shield extended out. 
Fold the zipper shield out of the way (as shown).
With the right sides together, line up the right-hand side of the zipper tape with the finished edge of the LEFT FRONT fly extension. Use a zipper foot to stitch the zipper to the FLY EXTENSION ONLY. Make one line of stitching close to the zipper teeth, then make another closer to the edge of the zipper tape.
Turn the pants so you are once again looking at the right side. Use chalk or tape to mark your fly topstitching. Mark the topstitching 1" (2.5 cm) away from the folded edge, down the fly extension, and curved in toward the top of the crotch seam.
Fold the zipper shield out of the way once more – be very careful not to catch this during the next step.
Following the markings, you made in the previous step, topstitch along the LEFT FRONT catching the zipper and fly extension underneath. 
Fold the zipper shield back under the zipper to its intended position and bar tack for 3/8" (1 cm) through all layers where shown. This area is often under a lot of pressure on casual pants.
UNZIP THE FLY ZIPPER and make sure the zipper pull is farthest away from the top of the pants.
Switch back to your regular presser foot. Within the seam allowance, at the waist edge, stitch horizontally over the zipper AND zipper teeth several times to create a new zipper stop. Do this on the LEFT FRONT and RIGHT FRONT. Test the new zipper stops by closing the zipper and making sure the zipper pull does not go past the stitching.
Trim off the excess zipper tape.
You have successfully created a zipper fly! Do a little celebration dance for getting through all these steps. 
OPTIONAL: Now that your fly is finished, press the seat seam allowances to one side. Topstitch the seam allowances in place to create a fake flat felled seam. To make this look even more convincing, you could edge stitch along the seat seam as well.
June 23, 2022 — Morgan and Matthew Meredith