There are many ways to sew an invisible zipper. The one we're sharing today is great for beginners since it helps you line up both sides of the zipper correctly. If you have seams running across the zipper this will help you line those up, too.
An invisible zipper foot makes things very easy. It has two special grooves that the zipper teeth will fit into. You can get by using a regular presser foot if you first press the zipper crease open.
Make sure your needle is centered and the stitch is set to straight, regular length.
Your edges will need to be finished before inserting the zipper.
First, do a basting stitch (longest stitch length) along the seam where the zipper will be inserted. Press the seam open. Now we know exactly how the seam will look after the zipper is inserted. If you're working with a dress and have seams running across make sure to match those now and they will match after the zipper is inserted, too.
Place the zipper on top, right side of the zipper facing down. Remember that the right side of an invisible zipper is the one where the teeth are not showing. Line up the center of the zipper with the seam and pin zipper to seam allowance only (not through all layers) Use plenty of pins to make sure the zipper doesn't move.
We will be adding a facing on the waist later so we are leaving 1 cm (3/8") seam allowance above the zipper.
Remove the basting stitch an unzip the zipper. Sew both zipper tapes to the seam allowance. Backstitch at start and finish.
A common mistake is for the zipper seam to end up looking wavy. This happens if the fabric stretches when sewing. Make sure not to pull the fabric when sewing, and always add interfacing to light weight, slippery fabrics. Spray starch is another great way to keep the fabric stiff. Any fabric with even a small amount of stretch will definitely require some stiffening with interfacing, starch or both before attaching the zipper.
After sewing both sides of the zipper switch to a regular presser foot and sew the rest of the middle back seam. It's fine to leave a little gap between the seams, it helps hide the bulk of the zipper. Start a little bit higher than where the previous seam ended.
Finally, press the seam open from the right side. The zipper should be perfectly hidden in the seam.