First you need to cut your scraps. Use any fabric you like! I just really liked the grungy feeling from the black and white polka dots with florals. :)
Take out the seams of the legs of your jeans. You can hand sew your patches on if you dont want to take the seams out, but it is pretty easy to sew the jeans back up.
You can either pin down your patches to the pants or use something to fuse the fabric with the pants so that it doesnt move when you sew. I used fusible iron paper, but it kind of makes the patches stiff. I would use dots of fabric glue or something and let it dry and then sew.
After you stitch around all of your patches, you just sew up the side seams of your jeans and you are done! Easy project. :)