I've made 2 types of denim quilts. One was made from the widest strips I could cut from jeans without getting any worn places like the knees. Then I took a large square and sewed the strips around it, kind of log cabin style, sewing strips end-to-end to get length when needed. Use a jeans needle in your machine. I used a 1/2 inch seam, and I pressed it open instead of to 1 side to avoid too much thickness. I backed with flannel, and the batting was thin cotton/poly. Simple straight line quilting, and bound in flannel. This was about 45 x 45, a baby quilt for my niece. I would do one like this again, turned out to be very cozy and durable. If you look at my profile, I have a picture of it in my quilt album.
The second one was a circle jeans quilt. I didn't really like the way that one turned out. It was okay, just not very cuddly, more stiff, a good "throw on the ground" blanket.
I made sure to use regular cotton jeans, not the newer blends that have stretch.