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.
|My overlocking foot|
The ruffle is one long strip of fabric. Fold it in half lengthwise (along the grainline) with the "right" side in. Sew a straight line along each short side (the non-overlocked side). Then, trim the corner off, iron the seam, and turn the ruffle piece right side out.
|The long edge is finished so it won't fray. Pin the short sides together.|
|Sew the short sides|
|Clip the corner|
|Turn right side out|
|Press the ruffle side seams that you just made!|
Ok, your ruffle piece is ready to ruffle now! You're going to gather the overlocked edge. This means you should go back to a normal presser foot and a normal straight stitch. Set your stitch length as long as it can go (5.0 on my machine). Sew a straight line, without backtacking at either end. The instructions say to sew two straight lines, but I'm lazy and just did one. If you have a thicker fabric, two would be better. Now, pull on your thread at the end. The fabric will gather up. Pull from both ends and distribute your fabric evenly until the gathered side of the ruffle is 21" long.
|The longest stitch on my machine|
|See the threads I'm pulling over to the right of the picture?|
|Gather to 21"|
Now attach your ruffle! You should pin the ruffle along all the stitching lines you made first so you know your gathering is distributed evenly and your ruffle is the right length. Once that's done, I usually tie the ends of the thread I pulled so it can't shift anymore. Now unpin down to the first little curve (from point A to B). Sew along that line. Then flip the fabric around so it faces along the B to C curve. Sew that curve. Continue until you reach the last point (F).
|Line up the ruffle along the lines--it's hard to see the lines in this picture|
|Pin the entire ruffle in place|
|Sew from A to B, right near the edge (not a 5/8" seam)|
|Sewing the ruffle|
|The "wrong" side of the fabric after sewing from A to B, B to C|
|The "wrong" side of the fabric after sewing the entire ruffle|