The ruffle seems to puzzle a lot of people who are making this dress, but it's really pretty simple! I think the point of confusion is that it's not an actual pattern piece, but since you've already cut it out, it shouldn't be too complicated.
You're going to use your handy overlock stitch that you used in the reusable grocery bag practice sew along. Finish the long raw edges. There is a special presser foot for this stitch. If you don't have an overlock stitch, it's ok, just zig zag along the edge to close the open edge. If you have no idea what I'm talking about, check my reusable grocery bag practice sew along post.