Since the shirt already fit me, I didn't have extra fabric to work with so I used the scraps of a jersey fitted sheet. (I promise it is a new sheet - never slept on! My Mom used it to cover some couch cushions and had leftovers)
To get the ruffle to go down the middle of the shirt, I pinned the right ruffle down first, then flipped the left one over so that the gathered edges were on top of each other. (Sorry there are no pictures of this part! My kids were waking up and I had to hurry!) When everything was pinned, I changed the stitch length back to normal and sewed around the neckline first, then down the middle. To make the left ruffle lay flat, I flipped it back over and topstitched it down. And here it is!