I started by measuring the length and diameter of the pillow. The width of my cut of fabric was the diameter plus one inch for a 1/2 inch seam allowance. Once I measured the length of the pillow (just to the edge seam), I added a few inches to each end to allow for an elastic pocket and to allow the fabric to go to the center of the end of the pillow.
Here's the pillow I started with:
After cutting one piece of the pink fabric, allowing for the measurements described above, I sewed the short edges (end) about one and a half inches back to make a pocket for the elastic.
I sewed the ribbons on by sewing down both sides.
I then sewed the long seam, but forgot to leave the ends open so I could insert the elastic. I had to rip the edges open...oops!
I measured the elastic by taking the length of the short edge and cutting it in half and using that size for each of the elastic pieces. The elastic is so that I can close the edge some but still allow the user to get the pillow in and out of the cover.
After inserting the elastic and pinning it in place, I sewed the ends of the long edge.
The finished cover without the pillow:
I wasn't happy that I could still see the end of the pillow once it was inside of the cover, so I turned it inside out and sewed Velcro inside to help it stay closed.
Once I got the pillow back inside and closed the ends with the Velcro, it looked much better!