To begin with, you need “PI”. What is that? To be exact, that is 3,141 592 653. What do you mean? That is how you measure the circumference of your pillow. The number is the ratio between the circumference and the diameter of a circle. Of course you can also use your centimeter.
My circle is 16 cm in diameter, plus 1 cm extra stitching. And that time PI 3.14 x 18 cm = 56.52 cm. And then an extra 1.5 cm on both sides for the zipper cover.
My inner circle is 4.2 cm in diameter. And that time PI 3.14 cm = 13.19 cm. And then another 1 cm extra for the overlap. If you can still follow it, this is about the rectangle flap that you stitch on the side (circle).
Lots of fun. The explanation is the subtitles that you can turn on and off at the bottom right of the video.