First, I used another skirt that fits her to measure the elastic and pinned the elastic so I'd know where to sew it together.
I didn't take a picture, but before this, I had made another, wider rectangle from the striped fabric, and sewed a pocket for the elastic to go on. I didn't thread the elastic through until the end. Next, I sewed the hemmed fabric onto the base fabric using a regular stitch. I first sewed it right sides together, then as you can see in the picture below, I folded the fabric over and sewed along the top of the seam to keep the ruffle down.