I had saved a picture of that finished project. Was needing instructions.
This is the only similar tutorial I could find https://www.hometalk.com/36411234/how-to-make-a-simple-window-seat-cushion-from-reclaimed-denim-jeans
This tutorial might help: https://betweennapsontheporch.net/101-things-you-can-do-with-old-jeans-no-4-upholster-a-chair/
Hi Claudia: Here is a site that outlines the steps. She has several blogs to look at: https://www.theshabbycreekcottage.com/make-slipcovers-part-1.html
Or, you can check with Pinterest.Com, they have a zillion ideas https://www.pinterest.com/pin/493707177882763194/, or just go to Pinterest.com and type in denim sofas.
If your couch still has the fabric on it, carefully take it off and use for a pattern. I would sew the denim together in patches to make a large piece of fabric, then lay your old fabric out and cut leaving a 1" edge all the way around. Then sew together and cover. I have never done a couch, that sounds interesting. Hope you post pictures.