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 phos shuld illustrate the rest of the process:
Apply zipper, as shown, to fly, matching edges of fly and zipper tape.
Apply fly facing to right front, matching raw edges as shown.
Stitch fly facing to right front with 1/2" SA. Turn under and press facing down, folding the right front at the marked CF line.
The left front should look like this from the right side
Snip at notch of left front almost to the CF marked line.
Fold left front down 1/2" - 1/8" away from the CF.
Baste left and right fronts together to about 1/2" below the zipper notch.
This is what you should have at this point.
Baste the fly to the left front.
Machine stitch the fly to left front.
Baste together left and right fronts so that the CF markings meet.
Baste the right side of he zipper tape to the zipper facing for the right front - only to the facing, not to the front piece.
Stitch down the zipper to the facing.
On the right front mark the stitch line. 
Stitch by machine on the marked line making sure you do not catch the flyStitch by machine on the marked line making sure you do not catch the fly. 
See - the fly is not caught in the seam:)
Machine-tack the zipper tape about 1/5" below the fly and cut under the tack.
And... You are done:)
Hope some will find this helpful and please ask if you have any question!
