Here's my 2 cents, I just made a baby raggy and a couple of purses.
1) Use heavier thread to hold the denim together. Do a trial block, wash it and dry it to make sure the thread is good for holding it after cutting.
2) Double stitch the seams joining the blocks.
3) Remember flannel shrinks with washing, I did a design stitch in the middle of the blocks to hold the flannel and cotton to shape. It looked really good and it gave body to the blocks.
4) I stitched around the block itself after doing the decorative stitch to give myself a depth cutting line. If cut to deep it will cause the blanket to come apart.
5) It makes a most beautiful quilt. My niece loves it and the more it washes the more it rags.
This is my results, I used a machine quilting thread to sew my first trial blanket together, it was old when I bought it and it pulled apart when I washed the blanket the first time. I had to restitch the seams. VERY frustrating to say the least.
I love the ease of the rag quilt and I'll make many more. Very quick to do (ok, except for cutting) and I highly recommend the correct scissors, if you have arthritis, get the ones made with the better handles and the spring for sure. I couldn't have cut so much without buying them. The best $15 I ever spent.
Happy Quilting, (Be sure to post a pic!)