There are lots of ways you can go about gathering the skirt of a dress to the bodice or a ruffle to anything, so I thought I'd share with you the easiest way we have found to do it. Sandi has a great tutorial here as a starting point, but we've taken it just a step further for you.
When you have a large circumference to gather up it can become unwieldy and difficult to get it spread out evenly all the way around. Here is what Amy and I do:
1. Start by marking (with your water-soluble marking pen) half-way points on your bodice, between the side seams, until you have 8 marks all the way around.
2. Do the same with the top edge of the skirt. I used to do this with large pins, but inevitably they would fall out in the process.
3. Next, gather up the skirt. I pull it all from one direction, securing the other end with knots. I usually just sew one basting stitch just inside the seam allowance, but in this case I had already attached the apron on this top and felt it was safer to use two basting lines in case just one would break.
Now you can match up those 8 marks and pin them together (skirt to bodice). Don't forget to have right-sides together.
4. Now spread out the fabric between those pins to get it even.
5. Then add more pins in between to keep it from sliding around while you are sewing.
6. Here is where I quickly turn it right-side out to make sure the gathering looks good and that I didn't miss any obvious spots.
7. Now you are ready to sew your seam and serge it. It's a good idea to cut the basting lines in case you gathered it a little tighter than your bodice. You'll be able to feel what I mean if it's a problem, it will be obvious. Also make sure they aren't visible from the top side. If they are just use your seam ripper to remove them.
8. Now admire your work and finish it up with top-stitching or trim.
And try it on your sweet girl.