The blogger Pillar Box Blue is where I go to for everything involving jeans and denim. Here's her link for a rag rug: https://www.pillarboxblue.com/blue-jean-rug-9-unique-ways/
Here is another tutorial: https://www.littlehouseliving.com/how-to-make-rag-rugs.html
Rag rugs I've made start with strips of your fabric of choice braided together. As you braid, weave in new strips to keep the braid going. Once you have a long braid, spiral it around itself, tacking with thread as you go and keeping the braid flat. (in my opinion, this is the hardest part)
Here is a great one! https://mypoppet.com.au/makes/upcycled-denim-rag-rug-diy/
You can either make a continuous 1" cord and Knit or crochet or plait with it. (knot or sew joins.
You could make a rag rug using canvas/sacking as the base, and cut out identical oblongs and using a latch tool push them through the sacking doubled up and thread ends through loop on good side of rug (makes a shaggy rug). Enjoy the process.