You will need 2 old pair of jeans.
Start by cutting the center crotch seam out, it is usually the thick one. and you can cut the top off by the front pocket and the rear pocket unless your waist size is bigger than 36, then you can use that area.
You will need for
Body of bag 14"X37" from denim
two sides 8"X15" each from denim
one strap 4"X24" from denim
then from a fun fabric cut
2 1" strips by aprox 37
1 3" strip by at least 42
1 4" strip by 24
I start by cutting off that thick seam and depending what size of jean and the condition of it I cut out all the bad areas like torn holes in knees etc, and most of the time the pocket area. I have used the pockets in the body of the bag when the waist size was at least 36. Take one of the legs and make your body, you may have to piece to get a 14X37 inch rectangle. then with other leg cut your 8X15. I usually do not have enough from one pair to make the straps so I use the second to cut the straps from one leg. but if you do you can also piece to get the 4X24 also. When ever I pieced I just ironed the seam open and sewed it down. you can do it all with denim too, If you want all denim and no fun fabrics do not cut the one inch strips.
Once all cuts out
(for piping) fold the one inch fun fabric in half and press and sew to the 37" sides on both of the body of the bag.
Mark the center on the body of the 37" both sides, and then on the side pieces just one side mark the center of the 8" side. Put right sides together matching at the centers, sew 1/2 seam. Then turn it, matching ends and sew the 45 degree, up all 4 sides.
Measure the top of your opening of the bag it should be about 42. take the 3" fun fabric and fold to make a 1 1/2 inch strip right sides out. and measure and sew the ends to make a ring the same size as the top measures. sew it to the top of the bag with the fold edge to the bottom of the bag. (like doing binding on a quilt) fold over the seam once sewn and sew it again to attach the other side to the bag encasing the seam.
For the straps put the fun fabric and the denim right side together and sew a 1/2 inch seam on both sides. cut it in half after sewn and press it open and then press in 1/2 and fold it over on it self so all edges are inside. sew along the very edge of the strap. hold both together and cut off rough end so they are both the same. attatch to the bag about 2 inches from the side seam ... Ta da you have a bag...