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.
Step 4: Flip the Bodice Inside Out.
I carefully pulled the back pieces through the straps of the dress in order to flip the whole bodice right side out. I pressed the sewn edges to get the nice, crisp look that befits a superhero.
Step 5: Sew the Side Seams.
Once the bodice is flipped right side out, line up the sides by matching the lining pieces right sides together, flipping the fashion fabric up and lining the sides up. Make sure that the seams joining the fashion fabric and the lining (on the bottom of the armscye) match up. Stitch a straight line to join your bodice sides; your bodice should now be ready to insert a zipper. You will need to shorten your zipper length to accommodate the cutout. My pattern originally called for a 22" zipper; I used a 14" one.
Step 6: Continue With Your Dressmaking As Usual.
Once you attach the skirt portion to the bodice, you can insert a zipper as you normally would. To attach the little strap at the top, I just flipped the edges in and sewed on a hook and eye.
And then you're done!