Edited to add: I've made a pdf file for this instruction. You can download this here.
Regularly I read that constructing a fly front zipper in a pair of pants is something that is considered difficult. It's not. There's a very good method by Sandra Betzina that can be found on the Taunton site.
Until my last pair of pants I used that method, but have now blended that method together with two other methods I found (one in Threads and one in Ottobre magazine). The advantage for me is that the facing of the fly is in this method from the start and that the front is not sewn together first, but only pinned.
I really recommend using a facing to prevent damage to your undergarments, but if you want, you can omit it from this. I use a zipper that is longer than needed, this is what I really like about Sandra Betzina's method, as you don't have to bother with the zipper stop, and it gives a very finished look when the waistband is added.
The pictures are made with a scrap of fabric, non matching zipper and thread.
1. Mark the center front by basting, add a fusible interfacing to the right (overlapping) side (picture shows wrong side)
2. Press the left side almost 1 cm (5/8 inch) from the center front (picture right side)
3. Stitch from start of zipper to inside seam, but leave enough room to stitch the inside seam of the pants later.
4. Stitch the zipper to the facing on the right corner to secure the zipper.
5. Stitch the zipper with facing to the left part of the pants, close to the zipper
This secures both zipper and facing to the pants.
Don't go down to the very end, leave a bit of space to fold away the facing later for the topstitching. I did it right by accident when writing this, but later did this part wrong and ended up taking out a few stitches.
6. Press the overlapping side on the center front line.
Pin it to the left side, matching center front.
7. Turn to the other side, pin the zipper to the fly extension, do NOT pin the fabric on the front
8. Stitch the zipper on the extention, again NOT the front.
10. Fold away the facing and topstitch on the front of the pants (sorry, I did not use a guide or template, this is a bit wider at the bottom, but you see what I mean)
The inside, I could have made the facing a bit shorter.
Important: do not cut off the extra length of the zipper until you have sewed the waistband. I did this once, and ended up starting all over again!
Edited to add these pictures, I realised that I forgot to make a tack on the bottom part (as often seen in RTW). This tack fastens the facing.
On the inside I added an extra tack to secure the facing (and yes, the facing is shortened too).