Then I cut a piece of fabric to make the "belt" portion. It was between 4 and 5 inches, I believe. I made it long enough to tie in the back of the shirt.
Then, I folded in the ends, so I wouldn't have any raw edges on the finished belt. I sewed the edges down using a straight stitch.
Then, I placed the belt right sides facing each other and folded the fabric in half. Next, I sewed the long edge shut with a straight stitch.
Once the fabric was a tube, it was time to turn it right side out. I used a fabric turner to make life easier.
After turning the fabric. I sewed the short ends shut to make it look more finished. Then, I pinned the belt to the front of the shirt in a location that would be right under the bust. I sewed this from side seam to side seam, using a straight stitch.
Next, I sewed the belt down "in the groove" of the side seams, so things wouldn't move around too much. After that, I just had to tie the shirt in the back. (My belt length got a little wonky on one side, so things aren't even. Oops!)
The true test was putting it on myself because my mannequin and I are not the same in the bust. Thankfully, the belt fit where I wanted it, right under the bust line, which made the shirt look a bit more form-fitting.