The instructions for sewing the Jutland Pants Fly Zipper may differ slightly from other patterns you have sewn. Some sewing patterns will have you sew up the fly at the beginning of the sewing process to work with flatter pieces and less bulk. The Jutland Pants, on the other hand, have you sew the fly near the end of the sewing process to allow for easy application of the cargo pockets and a little bit of a sewing warm up before you tackle this involved step!
Sew the Seat Seam
Finish the seat seam and fly-facing edges before you sew up the seat seam. Options include serging, zig-zagging, or binding. If you use a bulkier material, you may want to avoid binding for the seat seam and use the alternative methods instead. Binding is an excellent option for lighter-weight pant fabrics. 
Turn the pants legs so that one is right side out and the other is wrong side out.
Place the leg that is turned right side out inside of the leg that is wrong side out. Align the legs along the front and back seat seams, and align the inseams. Pin.
Stitch the seat seam together at 5/8" (1.5 cm) starting from Center Back and sewing towards the front. Stop stitching 1/2" (1.3 cm) below the zipper placement notch (the horizontal notch on the fly extensions). Check that your inseams line up nicely, and stitch over the entire seam 1 - 2 more times for added reinforcement. This area of the pants gets a lot of strain and needs to be sewn very strongly! Use a shorter stitch length while stitching this seam for added strength. Wait to press the seam allowances. 
Fold the fly extensions on the front pieces to the wrong side according to the notches.
The Left Front (as if you were wearing them) will be folded and pressed at the notch farthest from the curved edge. 
The Right Front (as if you were wearing them) will end up being behind the Left Front, creating a smaller facing and a 1/4" ( 6 mm) extension. Fold and press the Right Front extension at the notch closest to the curved edge.
Fold the zipper shield in half lengthwise, with the wrong sides together. Bind, zig-zag, or serge the long curved raw edges together.
  We will now be fastening the zipper to the fly shield. You can secure the zipper in place on the fly shield in a couple of ways. One way is by using pins. For this method, with the zipper teeth facing the right side up, match the left edge of the zipper tape with the curved edge of the fly shield. Pin along the left-hand side of the zipper to keep it in place.
My (Adrianna) preferred method of securing zippers in place for stitching is by using wash-away tape. The wash-away tape acts like a double-sided sticky tape that will disappear after washing your garment.
First, trim off a strip of tape from the roll about the length of your zipper to use the wash-away tape. The wash-away tape is relatively sticky, so if it extends over the zipper's edges, it may start sticking to other parts of fabric or threads and get in the way. 
Flip the zipper over so the wrong side of the zipper teeth are showing. Place the sticky side of the tape along the right-hand side of the zipper. Using your finger, press the tape along the zipper so that it sticks.
Carefully peel the paper away from the tape to reveal another sticky side!
Flip the zipper teeth to be facing right sides up. Place the zipper's sticky side with the fly shield's curved edge. Using your finger, press along the zipper to stick it in place.
**If you used the pinning method, baste the zipper to the fly shield using a zipper foot.
If you are using wash-away tape, continue below.
With the zipper and shield facing right sides up, cut another strip of wash-away tape about the length of the zipper. Place the sticky side down on the left-hand side of the zipper. Using your finger, press the wash-away tape onto the zipper so that it sticks.
Carefully peel away the paper from the wash-away tape to reveal another sticky side.
With the right sides of the fabrics and zipper facing up, match the zipper and fly shield to the right front of the pants (as if you were wearing them). Position the folded edge of the center front about 1/4" (6 mm) away from the zipper teeth. 
Press along the center front edge using your finger so that it sticks to the zipper and fly shield.
Using your zipper foot, edgestitch 1/8" (3 mm) away from the folded edge through all front, zipper, and fly shield layers.
Now unfold the fly extension on the left front. Position the center fronts of both legs to match the right sides together. 
With the right sides together, match the unsewn side of the zipper with the fly extension on the left front.
Using your zipper foot, stitch the zipper to the left front. Make one stitch line close to the zipper teeth. Make a second row of stitching right along the outer edge of the zipper tape. 
Position the pants facing right side up to look at the center front. Refold the left fly extension and see how the fronts and zipper lay. Check to ensure the fabric is laying flat, and there are no tension, puckers, or folds in the fabric. 
Fold the zipper shield out of the way. Pin the left front fly extension in place.
Using the template, draw a J-stitch line on the left front.
Stitch along the J-stitch marking. If you want to create a double row of stitching to match the topstitching on the front pockets, make another row of stitching 1/4" - 3/8" (6 - 10cm) away from the first one.
You can either echo the same curve the entire length or taper the curve so that it ends at the same point as your first line of stitching.
Finish Seat Seam
Press the seam allowances of the seat seam towards the left leg (as if you were wearing them). Topstitch along the seam through all layers, 3/8" (1 cm) away from the seat seam line. If you made two rows of stitching on the front pockets and J-stitch, make another row of stitching 1/8" (3 mm) away from the seat seam line. 
For reinforcement, make a bar tack along the bottom edge of the J-stitch where the seat topstitching, J-stitch lines, and zipper opening meet. 
October 11, 2023 — Morgan and Matthew Meredith