One time I went to buy a zipper in the Garment District in New York and could only find invisible ones, not the regular ones. I asked the old Russian guy if he had any regular ones, and he said nobody uses them anymore and told me I should ask the girls in my sewing circle to show me how to insert an invisible zipper. I said I wasn't in a sewing circle and I didn't have an invisible-zipper presser foot anyway. He sighed loudly, then proceeded to teach me how to do it with a regular presser foot. I've now done this a dozen times or so, and though I don't claim to be an expert zipper installer I thought I'd give a little tutorial here, because nine times out of ten the zipper comes out looking great—that is, you can't see it. So here we go. (I hope you can make out what's going on in the pictures. I tried to get them as clear as possible, but it was difficult with black on charcoal, and the moiré effect I kept getting with the wrong side of the knit material.)
Do not sew the garment opening closed as you would when inserting a regular zipper; leave it open. Open the zipper, lay it out face down, and press the teeth flat (toward the center) with a warm iron. You should be able to see two rows of stitching right next to the edge of the teeth.
Pin the right side of the zipper to the right side of the garment opening, with the top of the tape at the top of the opening and the teeth at the 5/8-inch seam line.
Using a regular presser foot, sew the zipper to the garment, stitching as close to the teeth as you can without piercing them.
Sew it just like you sewed the first side (this time it'll be going under the presser foot backwards, but that's okay).
Close the zipper. Pin the rest of the garment seam, right sides together, below the zipper, letting the bottoms of the zipper tape stick out the seam so they don't get caught.
Here's where it gets a little tricky. Place the presser foot as close as possible to the backtacking at the end of where you sewed the zipper onto the lefthand (when it's inside out) garment piece. Set the needle to the farthest position to the right. Sew the seam.
Press the seam open and the zipper flat. I didn't have to do it this time, but you may have to take a few stitches by hand to close up the space between where you sewed the zipper on and where you started the rest of the seam.