Be careful of your hands when cutting jeans up! This is the type of job that can really do a number on your joints, tendons, etc.
What helped me was using a cordless electrical shears to cut off the seams and waistbands. I have an old one, but there may be similar versions on Amazon. You need the kind that can cut through heavy fabric. Mine goes through jeans fabric like butter!
Even after I got the jeans fabric flat, I developed hand problems cutting it into squares with my rotary cutter and ruler. I decided it's not worth damaging the joints in my thumb and index finger with this kind of project, so now I have a jeans UFO in the closet to add to all my other UFOs!