There are several variations and a lot of tutorials on how to sew a fly front in a pair of jeans/pants. This is my take on it, a combination of how I learned it long ago, instructions for Jalie jeans, a video tutorial by Jennifer Stern (which I can't find now) and just doing it for a long time now.
The steps that I show:
- Are for a pair of womens' jeans/pants, closure with the right side over the left side.
- Use a pocket that is extended to the center front, the steps for the pocket are not shown.
- Can be used with regular pockets, just follow the instructions except where I mention that
- The pocket extension has 0.5 cm seam allowance at the center front for the left and is cut at the center front without seam allowance for the right side
Fold the fly (it usually is a pattern piece that is cut on the fold) and finish the seams. Sew the zipper on top, stitching only the side that's closest to the finished edge. (My zipper is a bit longer than necessary, I'll cut it shorter after attaching the waist band)
Finish the seams of the front pattern pieces. I use a cut-on facing on the right side, the overlapping side on a womens' pair of pants. If you prefer you can use a separate facing, you'll have to sew that on as a separate step.
Pin the pocket/lining in place, the left side has the small amount of seam allowance as described above. (if your pockets are not extended to center front, omit this step)
Sew the center front until the point where the zipper will end.
Fold and press the right side on the center line. Make sure the pocket on the inside is aligned with center front. Topstitch till the crotch.
On the inside it looks like this:
I like to sew in the zipper a little off-center. Fold the left 0.5 cm beyond the center and press. Pin the zipper with the fly in place. Sew on the edge.
Pin center front of the right leg to the center front of the left leg.
On the inside, fold away the fly and pin the zipper to the facing of the right side. Only to the facing!!
Sew the zipper tape to the facing. The zipper is attached, the topstitching in the next steps does not have to be on the zipper tape, only on the facing.
I mark the end of my zipper with chalk, placing a pin to mark first. This way I know where the end of the zipper is. Especially important if you use a metal zipper, which would break your needle if you would hit in while topstitching.