I love zips, wonderful invention, I do like other fastenings too but zips are the most easiest and convenient for me. The only problem with zips, more specifically concealable zips is that when they run through a waist seam it can be difficult to mach the seam up, so this is how I do it and the technique used on yesterdays post .
Sew the one side of the zip to the fabric as normal.
(note, I missed step 2 out and couldn't be faffing about with Photoshop to correct all the steps!!)
After stitching the one side of the zip, pull the zip up so it is closed.
Pin the fabric and zip you were holding in place and proceed to undo the zip.
pin the zip properly as you would if you were sewing the zip.
do the zip back up and see how well it lines up to the seam hopefully it should match perfect but if it not adjust accordingly, from here you can either keep it pinned in place which is what I do, or you can hand/machine tack this area of the zip in pace for added security.
et voila, the seam that the zip runs through should now match up perfectly.