Disclaimer: I didn’t use a pattern piece. I should have. One day I’ll learn that things turn out wonky when I just quickly eyeball it and cut what I think it kinda sorta pretty much looks like. Yeah. One day.
Sew the crotch seam to the zipper opening (marked here in blue tailor’s chalk)
Clip at the zipper opening (bottom of fly extension)
Press the fly extension along the center front line to the wrong side of the fabric
On the left fly extension, slide out and pin 1/8 inch from the pressed line. Do this to 1 inch beyond the stitched seam.
Pin the closed zipper to the left fly extension, with the correct sides up (my awesome “wrong side” markings bled through to the correct side. awesome indeed.)
Sew from the bottom of the zipper to the top, close to the folded edge
With correct sides up (again with the bleeding of the markings. so sorry) pin the other folded fly extension over the closed zipper and the stitching
Wrong sides up now. Make sure you fold the rest of the garment out of the way. You are only going to stitch on the fly extension and zipper tape
Stitch the other side of the zipper tape to the other fly extension, the entire length.
Turn the project over, stitch 3/4 inch parallel to the fly fold through all layers: fabric and zipper tape. Curve it at the bottom to meet the bottom of the zipper
That’s it. Not too bad.