Putting a zipper into a dress is a technique that requires a bit of precision in order for the dress to look good and the zipper to operate smoothly. The zipper must be centered exactly over the back seam of the dress, with stitching that is evenly spaced on both sides. Sewing it in without careful preparation will likely result in a zipper that peeks out from gaps on the back of the dress, or one that snags on stitches that are sewn too closely to the zipper’s teeth.
If you are replacing an old zipper, remove it by using a seam ripper to break the threads holding it in place. Be careful not to rip the dress fabric. Loosen the hem facing the neckline. From this point on, the instructions for putting in a zipper are the same regardless of whether the dress is newly constructed or the zipper is being replaced.
Baste close the seam where the zipper will be placed. You can do this either by hand or machine, using long running stitches. Do not make these stitches too small or tight, as they will later be removed.
Press the seam open, using the point of an iron to flatten the seam so that the edges of the fabric lie flat on both sides of the seam. Make sure that the fabric does not fold or wrinkle over the center stitching.
With the inside (wrong side) of the dress facing you, lay the zipper face down over the seam. Place the top of the zipper at the neckline and center the teeth directly over the seam. The tab at the top of the zipper will face the outside of the dress. Pin the zipper in place.
Using a hand needle and thread, use long running stitches to baste the zipper in place. Remove the pins.
Put a zipper foot on your sewing machine. A zipper foot is narrow and can be positioned on either side of the zipper to allow you to sew close to the zipper's teeth and past the zipper tab.
Turn the dress over so that you are looking at the outside of the dress. Sew the zipper in place, stitching ¼ inch away from the teeth. Keep your stitching as straight as possible. Sew down one side and turn and sew across the bottom of the zipper below the teeth. Go back up to the neckline and sew down the other side, turning at the bottom and ending by overlapping a few stitches at the bottom where you ended your first line of stitching. Sewing the stitching lines in the same direction, rather than turning at the bottom and going back up, prevents puckering.
With a seam ripper, remove the basting stitches holding the zipper in place and the basting stitches keeping the seam closed.
Finish the neckline by folding over the facing or making a narrow hem. Sew a hook and eye closure at the neckline, at the very top of the zipper, to keep the top of the zipper together and provide a smooth, finished look.