This is a technique that I've used forever and have recently documented. I did not invent it, rather it was reverse-engineered from several high-end RTW pants.
Start with cutting your fly piece and your fly facing piece:
- Fly piece: Line it with stable lining fabric, pocketing fabric, or other lightweight cotton fabric. Lining is done by cutting another piece from the lining fabric and sewing it to the curved part of the fly. Turn to the right side, press and edgestitcg along the curve. Serge the raw edge. Trim SA of fly to the width of the zipper tape you are using.
- Facing piece: Interface the facing. Serge or bind the curved edge of the facing. You do not have to treat the remaining raw edge in any way. Trim the SA to 1/2".
Mark the CF 5/8 SAs and the zipper notch on both front pieces.
The following slideshow documents the rest of the process: phos shuld illustrate the rest of the process:
You can also view the entire tutorial in a non-slideshow format on my blog http://theprincessseam.blogspot.com/