How to Insert a Fly Front Zipper
Place the zip face down lined up the basted seam. Pin it along the left tape to the extension only. Then using a zipper foot, stitch the zip to the extension, very close to the teeth.
Now you can attach your waistband or facing and you're done.
Thanks to Sandra Betzina whose free online video of making a fly front, and brilliant book (Power Sewing, Taunton Press) taught me this method. You can also find a similar method in SewStylish magazine (the jacket issue from 2010).