Below you can see how I made the ruffle. I actually left the edges on both sides of it raw, meaning they are cut and not hemmed under or anything. Makes it really easy that way and I like that old fashioned handmade look. To learn how to make this ruffle easily on your machine go HERE.
Below you can see how much I gathered it to fit the rectangle edge. Then I pinned the right sides together, also shown below, and then sewed them together.