And, just saying; I am a believer that there is no necessarily right or wrong way to do anything in sewing, but whatever works and is neat and tidy is pretty much going to be a "right" way, no matter how you got there! This is merely just showing a way that I chose to do it :)
Step 1; sew the zip into the open seam. Also, when inserting a zip into a very lightweight sheer fabric like this I always stabilise the seam with strips of the selvedge as described here, the ends of these can be seen at right protruding out either side from underneath the seam. I used the selvedge of the thicker lining fabric here.
On the inside, pinch the seam allowances together, aligning the raw edges, right sides together. Snip 6mm (2/8") into the seam allowances, underneath the zip tape, about 1cm away from the stitching.
Trim the seam allowance to half its width.
Turn to the inside, folding as accurately as possible along the line of stitching with wrong sides together. Press the fold firmly in place.
Baste a 6mm (2/8") seam, commencing a few inches below the lower end of the zip and going up to as close to the zip stitching as possible. Turn it out and check for straightness and neatness.
Note; this is the most difficult part of the whole thing! below at right is pictured my first, not particularly straight, line of basting stitches :P
If it isn't neat, unpick and re-baste. When you're satisfied with it, stitch over the basting stitches.
