I did not measure. I simply unzipped the pillow covering that came with the store-bought pillow and laid that down on top of my brown flannel. It served as a pattern.
Then I guessed at how much "ruffle" it was going to take and thanks to the Lord and His answering prayer it came out right. The photo will show the ruffle laying on the table.
Then I gathered the ruffle and pinned it to the FRONT of the pillow. It is pinned right side to right side--the photo makes it look like it's pinned to the wrong side. It's only this way so that you can see it well. The bigger "quilt" pins will work far better than dress-maker pins.
I cut 2 pieces for the back. They are much wider than the front and hemmed so that I can remove the pillow easily for washing. The flap is wide enough that it will not show any white pillow underneath.
After you have the ruffle sewn to the front it's time to sew the back on ONE piece at a time and then you can't make a mistake.
I did not follow any directions or pattern. I just prayed.