Start by cutting the leg off the jeans at about the knee. Cut the hem and the seam up the side of the leg off too, to make the jean material easier to tie and use.
Cut an old t-shirt into the same size strips as the jean. If you have a large dog, you can always skip the cotton and just do three strips of jeans instead.
Really tug on it to make sure it’s tight, otherwise, your dog will unravel it too easily. Then braid the three strands tightly and then tie the end in a knot tightly.