Cut your dress apart vertically, straight down the center of the back from neckline to hem. Don't worry about splitting the neck and hem, since the raw edges will later be incorporated into the zipper. If you are making a new dress, these are the center back edges indicated on your pattern.
Match the vertical center back edges together with the right sides of the fabric touching. Pin the edges together, starting at the hem and stopping a few inches below the hip. Sew the pinned length 1/4 inch away from the edges. This is the center back seam.
Measure what's left of the open center back edges. This length should begin at the top of the dress and end at the top of the center back seam. Find a zipper in this length.
Fold the open edges of the center back, including the edges of the neckline, 1/4 inch to the inside. Iron the fold.
Slide the left side of the zipper tape under the left fold. Pull the fold over the left half of the zipper. Pin the tape to the fold all the way down the zipper.
Slide the right side of the zipper tape under the right fold. Pull the fold over the right half of the zipper, meeting the left fold. Pin the right tape to the right fold all the way down the zipper.
Attach a zipper foot to your sewing machine. This will allow the zipper to move under the foot without getting caught. Sew the zipper tape to the dress on each side, as close to the zipper teeth as you can get.
Sew horizontally across the bottom of the zipper tape, anchoring it to the dress just where the zipper ends.
Trim the top edges of the zipper tape so that they are not visible. Slip into your new, easy-to-zip dress.
If your dress has a lining, slide the zipper tape between the outer fabric and the lining in Step 5.
For a side- or front-zipping dress, perform the steps on the side or front edges instead of the center back.
You will not be able to install a zipper in a dress that is already tightly fitted to your body, since there is no room for a seam allowance. Use a new pattern, a dress that already has a center back seam, or a dress that is loose enough to lose 1/2 inch.