Get an old pair of jeans, and cut along the dotted line in the picture. Make sure it's as straight as can be. I used a ruler and a permanent marker, just to be sure. :)
Make sure you don't JUST cut the legs off! You need to cut the entire bottom off. (above the legs a little)
Once you've cut that, you'll need to sew the bottom, where you cut. Make sure it's inside out! :)
<---It should look like this on the bottom.
Turn it right side out again. Now you have the main bag! :D Now you'll need to pick out a strap! I just used scraps from the jeans as a strap, and tied it to the belt loops, but there's lots of other things you can do... (Example: You could use a belt, string, yarn, or anything really! Just make sure it's strong enough to hold the bag and anything you put in it.)
Voila! You have your homemade jean bag! :) Hope you like it!