I started out with this fabric from IKEA...
and my ruffles from rufflefabric.com...
I cut my fabric 3 ruffle rows wide. No need to finish the edges because this fabric doesn't fray! I wanted the ruffles to be even more ruffly so I ruffled the ruffled fabric...does that even make sense? To determine the length of the ruffle fabric, I estimated how many inches around the pillow would be (the diameter), and I used about double the length of ruffle fabric.
I did a quick hand stitch under the middle ruffle and scrunched the fabric.
Then I pinned my ruffle in place on the front and back of the pillow and sewed it down. I just used a straight stitch right next to my hand stitch. Once the ruffle is sewn on, remove the hand stitch.
Finish the pillow like normal...just make sure the ruffles are lined up on the front and back before you sew the 2 sides together.
I love how easy it was to add some prettiness to a plain pillow!