Sewing: How I mended a duvet cover and added ribbons so the duvet wouldn't slip

I’m still working on mending some things that Lilo the cat ate before she was taking medicine for her fabric-eating compulsion. This project is a duvet cover that she nibbled near one of the corners.

I sewed a seam around the hole, at an angle that connected the side and bottom (using white thread, below).

I trimmed off the excess fabric, leaving about a quarter inch past the seam. Then I sewed down a piece of ribbon at the corner (seam and ribbon below) …

… and the other three corners, so I can tie the duvet in place to keep it from slipping the next time I use this cover.