HI,
I asked the original question and since then have finished the quilt and it turned out great! I actually pleated the ruffle first. Just turned the material under one half inch as I went along.
Then just like binding - I sewed completed ruffle on the backing using one-quarter inch from end ( be careful to just sew ruffle on to backing material and not the batting or front of quilt ) Then fold ruffle out and from the front of the quilt - fold batting and front quilt under ( at least one-quarter inch or one half inch) and stitch all along the border. I stitched close to the edge of the folded batting and front quilt so it looks very neat.
Hope this is understandable. And good luck!
Lizstitch

I pin it to the quilt and then pin the binding over top of that, raw edges even and stitch just like I would if I were only adding a binding to the edges. Then when I turn the binding to the back, the ruffle sticks out.

i've done as ruffle and also pleated... looks nice either way! Pics?

Did you have any trouble mitering the corners, or did you just round them?

I just kinda put the fullness so it looked nice (I pinned sideways about 4 inches before corner, then could "flip it outward" to check the fullness... went for visual vs. measured there!

For the corners, I just treated the ruffle like I was using binding on the corners. Stop stitch 1/4 from end. Fold binding/ruffle up 90 degree angle and then down and continue sticthing 1/4 inch from edge of material. ( I think this is the correct terms?)

What a life saver
 

I was trying to figure out how to put a ruffle on a baby quilt & just found your explanation. Thank you SO much for that! I used to watch my great-granny hand quilt, but never saw how she did prairie point or ruffles. You have saved me a bunch of time & mistakes! I appreciate it.