disclaimer: The following tutorial is how I changed a mock fly to a zip fly. It's not the only way and I'm not going to even say that it's the correct way. I based it off of Ottobre Design's zip fly because I use their patterns the most for pants that close with a zipper and that is what I am accustomed to. I'm assuming a bit of prior knowledge so if this is your first time sewing a zip fly, I highly recommend doing a trial one to practice.
Pants Pattern that has a mock fly
I used the Oliver + S Sketchbook shorts pattern. You need a zipper that is long enough for the size pants you are sewing. If you don't have a fly shield from another pattern, you can draw one.
Folded in half, the fly shield is the same width as the right section of the mock fly. The longest point on the bottom is 1" below the dot and the other is 5/8". You will be cutting out 1 fly shield.
Constructing the fly shield:
1. Sew bottom seam with right sides together with 3/8" seam allowance.
2. Trim that seam allowance to 1/4".
3. Turn right side out and press.
4. Finish open long edge together.
Set aside your fly shield for now and sew the pants inseams. Press seam allowances open and finish in your preferred method.
Go ahead and finish the raw edges of the pants' crotch including the mock fly. Then stitch the crotch seam stopping at the large dot (Sketchbook shorts).
If you don't already know what side you want the fly to open on, pick one. I grabbed a pair of P's shorts to compare. (Very scientific of me, I know) I could tell you that a man's fly should lap left over right while a woman's fly laps right over left but in my own closet I have pants/jeans from the same company that lap different directions. I chose left over right for P's pants. If you choose to do the other way, be aware that the instructions and pictures below are only showing how to do the left over right.
On the side that is not going to be covering the zipper, mark a line 1/2" from the original dotted line. Cut along the new line.
Finish the edge that you just cut and fold under a 1/4" seam allowance. Also, fold the other side on the original dotted line and press.
Now you will be sewing the zipper to the fly shield. Line up the side of the zipper with the long finished side of the fly shield. Stitch close to the zipper.
Next, place the folded under edge right against the zipper, lining the top of the fly shield to the top of the pants. Sew.
At this point, I folded over the other side to give me an idea of how the "lap" was going to work.
I then pinned the zipper tape approximately 5/8" in from the fold.
You will need to unfold the fold because you only want to stitch the zipper to the inside of the cover. That means you'll be stitching on the wrong side of the zipper.
I started with my zipper zipped and then once I had sewn 1/3 of the way, I put my needle down and lifted the pressure foot so I could unzip the zipper and move the pull out of the way.
This is what you should have now...
Zip your zipper to make sure that it was stitched on correctly and lies flat when zipped. If all is good, it's time to draw a stitching line along the edge of the lap cover.
*I apologize for the dim pictures. We've had a lot of dark and stormy days and I'm afraid my lighting isn't great.*
When you stitch along your marked line, you want to catch only the bottom of the fly shield. I started sewing from the top down and had the fly shield folded to the side until I got near the bottom.
Here's how it will look from the inside:
And from the top:
As far as the waistband, I cut mine about 3/4" of an inch longer to give me room if I needed it at the fly. I also interfaced one half of the waistband (on the wrong side) and considered that my outer waistband.
On the inside waistband, I folded/pressed the long side 3/8" to the wrong side. I then sewed the right side of the interfaced waistband to the right side of the pants with a 3/8" seam.
To finish the short ends of the waistband, I folded it in half right sides together and marked where it would line up on each end with the fly. Then I sewed the ends together, trimmed, and turned the waistband right side out.
The pressed under edge will be to the inside of the pants and should cover the stitching line. Pin in place and stitch. I chose to stitch in the ditch (mostly!) as these are to be dressier pants for my son. For a more casual look feel free to top stitch the waistband.
Pick a button and sew a button hole and voila! Zip Fly pants.:)
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