I put a side zipper in my
jeans as it seemed more 40s/50s period appropriate. However, I couldn't find any detailed-enough-for-me pictures of what one from that time looked like. Here's what I did, in more or less chronological order (some of the things I realized I should have done sooner or later, so the pics don't reflect that accurately):
1. Finish seam edges before sewing. The side edge of the pocket
is the front side seam, so finish that before making the pockets. I'm lucky to have a serger (compliments of
gorthx) so use that, but I've been making jeans for years and a simple zig zag stitch works just as well. I've never had any more problems with seams raveling with a zig zag than serging.
2. Sew side seam up to bottom of zipper insertion point and back stitch. I used a 7" zipper and it's
just long enough. A 9" zipper would have run too far into the pocket area where the seams are really bulky. Press under 1/2" seam allowance on the back and 3/4" seam allowance on the front.

3. Pin the zipper to the back, with folded edge along zipper teeth. Sew it!

4. Pin the under lap under the zipper with raw (finished) edge lined up with the sewn edge of the zipper tape. Stitch from the right side through all layers next to folded edge - yes, you will have two rows of stitching on the right side. I finally realized my stitching is curved on the wrong side/inside because the side seam in the hip area is curved. I might cut the underlap with a curved edge in the future. Not that it made a difference on the outside. Or maybe it did! Who knows.
I realize now looking at this picture the "under lap" may be confusing: I'm
calling the "under lap" the rectangular piece of fabric, so a better caption
for what's going on might be "The zipper is under the under lap". And
hey! the seams are now magically finished!5. Clip 1/2" into the back seam at the bottom of the zipper tape to facilitate folding the side seam under for flat-felling.

6. Edge stitch and top stitch side front opening down to the bottom of zipper opening.
I outlined the zipper tape in black in Photoshop
to distinguish the jeans
blue zipper from the jeans blue jeans under lap. Ha.
7. As for a regular lapped zipper application, pin opening closed over zipper matching center lines (ie, the front folded edge will extend 1/4" beyond the back folded edge). I pinned the lap out of the way this time (if I sewed through all the layers, I'd be sewing the opening shut!), sewed another 1/4" away from already existing top stitching (at this point 3/4" away from edge), then across the bottom to the side seam. The stitching across the bottom should be about 1/4" away from the zipper stop. You do
not want to try to sew through that on a plastic zipper much less a metal one.
Note: I would probably have had an easier time adding the underlap after this step. Somthing to try for next time.

8. Edge stitch and top stitch side seam (flat fell it!) to join up with the zipper top and edge stitching.

9. Unpin under lap and tack the bottom of the zipper.

I'm not completely enamored of this closure, but it was pretty easy to do and works well. What's your favorite side zipper closure? Or do you skip the zip and use buttons?
