This is my new favorite thing to do: embellish a little toddler Target tank top with a neck ruffle.
1. Get a strip of fabric about two inches wide, give or take, and twice the length from shoulder to shoulder of the tank top. It doesn't even have to be cut on the bias. Just a strip cut straight across the fabric works. (This looks just fine not going all the way around the back, I promise. I didn't continue the ruffle around because I knew my daughter's hair would probably hit right at the top of the ruffle).
2. Anyway, sew a basting stitch from one end of the fabric strip to the other, then pull your thread to gather the strip (or use your preferred method of gathering).
3. Pin one end of your gathered strip to one shoulder, and continue pinning along the neckline until you reach the other shoulder, adjusting the gathers as you go.
4. Then sew along the ribbed neckline. Et, voila! Seriously, this took 15 minutes only because I was watching the Tori & Dean marathon in the background. Let me know if you have any questions.