First, lay a piece of fabric over the top of your pillow insert and cut it to size leaving a good 1 inch of overhang around all the edges. On the ends where the ruffles go, I may have left 2 inches of overhang for more flounce.
Now you are going to cut the ruffles. You can choose any width you want. To determine the length, measure all around the open edges of your pillowcase and times it by 2, or in other words, double the length.