I am a very experience seamstress and I have been sewing for a really long time. I have been looking at the photo and I don't think rectangles of fabric will make a ruffle like that. It is more likely circles of fabric as there is no gathering where the ruffle joins onto the dress. It is hard to tell from the photo but it looks like there may be at least 4 circles making up that one ruffle.
To make the ruffle you would need 4 circles of fabric cut like a circle skirt, except that you would cut through the circle as i have drawn in my diagram. This will become the seam to join to the next circle. The circumference of the inner circle would be one quarter of the length where the ruffle joins the skirt plus seam allowances.
It also looks like that the ruffle is fully lined. I would use iron on interfacing on both my ruffle and the lining to give the stiffness that it requires. Hard to tell from the photo but it looks like there is more than on ruffle.
You are going to need a lot of fabric to accomplish this.