Zippers are often a source of trepidation for some bag knitters or even sweater knitters. They are avoided. . . but they should not be.
I have even seen knitters simply leave the zipper out, replacing it with a single magnetic snap. . . this is not a great solution. The inadequacy of this solution is clear when we imagine a knitted pillow on the couch with the pillow casing closed with a single magnetic snap. Immediately you can imagine the gaping that is created on both sides of the pillow. This creates a bad fit . . . and other readily visualized problems.
Setting in a zipper takes a bit of persistence, but it is not difficult and does not require particularly specialized tools. I, for example, have a wonderful sewing machine but do not use it to set in zippers. I sew zippers in by hand . . . with, as I always say during my workshops, “big, ugly stitches.” I can show you those stitches to prove it.
I have only twice had those big, ugly stitches come out. Once because the bag was stressed in a very odd way (not likely), and once because I used it so much and in such a rough way that I had to do over a short stretch of zipper and bag opening with new big ugly stitches.
Truly, anyone can set a zipper into a bag opening or pillow.
We begin with our materials list: work horse scissors that can cut through plastic, straight pins, thread, a sewing needle, and, of course, a zipper.
Once your zipper is the proper size, then make sure the bag/pillow opening is square–that is, aligned properly. Then place pins on both sides so that the two sides can be kept in alignment as you insert the zipper and sew it in place.
Sew in the first side of the zipper: use a running stitch on the inside but an invisible stitch on the outside. My recommendation: after bringing the needle out on the right side, then place the needle back in the fabric almost where you came out. All that will be seen on the outside is a small prick mark (if any mark at all). You can even use a thread color that is wildly different than your bag and it won’t show.
Once the first side of the zipper is in place, then zip closed and use pins to mark the zipper itself in direct alignment with the bag sides. This way, when you unzip the zipper, you can align the pins and pin the second side of the zipper in place in perfect alignment. Before sewing, zip the zipper closed to make sure the opening is, in fact, aligned. Make any adjustments before setting in place with your stitches.