There are tutorials online for making back cutouts in dresses with side zippers, but I couldn't find any for dresses with center back zippers, so I thought it through and tried it. I wanted a diamond cutout to echo the shape of Superman's logo, since my dress was made with Superman fabric.
You can make this modification with any fairly fitted bodice pattern that has a center back zipper. I started with the McCall's 5845 pattern. These instructions assume that you are familiar with the basic steps of dressmaking and will not teach you to insert the zipper or make the dress; I'm only explaining how to modify your existing dress pattern for the cutout.
Step 1: Prep your bodice pattern.
First, I sewed up the neckhole and armholes of my bodice, right sides together. Then I used a washable fabric marker to sketch the shape I wanted on the lining of the dress using a washable fabric marker. Make sure that you account for seam allowance. Note that I had the point of the diamond end where my center back seam will eventually be, not where the fabric ends.
You may find it helpful to sketch a shape on another piece of paper first, then use it to trace the outline onto your lining fabric.
I pinned around the cutout to hold the bodice and lining together, then sewed the outline of the cutout by following the lines I drew on the lining. Since my shape was all straight edges, it was pretty easy, just need to make sure to stop and pivot at the corners. If you are doing a shape with round edges, like a circle or heart, go slowly and rotate a little bit as you sew.
Step 3: Cut out the cutout!
I used pinking shears to cut out the diamond shape, leaving about 1/4 inch of seam allowance. I clipped the corners almost to the stitching line so that it would be a crisp corner once turned.