I followed the Pinterest link but was unable to find the original site or how the garment was created. So I had to figure it out myself. First, I cut a strip up the center of the back, curving outward over the shoulders, and then back under the arms in the front.
I cut a tapered piece of fabric out of each sleeve (both sleeves are sandwiched together in this photo) to become the back ruffles. It's the middle piece you want; the left and right pieces just went into my scrap pile.
Each tapered piece got hemmed on the ends and then folded in half lengthwise. I gathered them a bit along the straight edge, pinned them in place, and sewed them to the racer-back portion of the shirt. They were sticking out a bit more than I liked, so I went back and tacked them down in a few places with a couple hand-stitches.
Sure, I'm no model, nor is a camera phone exactly the epitome of high-end photography. But I think my copy-cat version turned out pretty nicely!