Cut your jeans off just at the crotch area. After you've removed the legs, tidy up the edges a little.
NB: DON'T BIN THE LEGS! WE'LL NEED THEM LATER ON.
Turn the denim, er, thing, inside out.
Grab your needle, and sew along the edge, WHICH IS NOT THE TOP! In other words, sew along the edge you cut.
You should do this at LEAST twice to make it strong, and use a stronger/heavier thread.
Now get one of the legs you cut off earlier. Cut along it until you have a long, thick strip of denim.
OPTIONAL: turn the edges over and sew them so you get a neater pattern. Repeat the last two steps, and sew (or, for you cheaters, glue) the other strip onto the underside of the first.
If you like the rustic, unfinished look, leave the strip as it is.
Sew the strips onto the sides of the bag to make a handle.
OPTIONAL: do this twice for two straps.
Now,cut a shorter strip from the leg. You can hem it like you may have done with the handle, if you like.
Sew it on to the back of the bag, opposite the button (your jeans need to be the type that button at the front-most jeans do, but jeggings don't. If yours don't-no need to worry, sew/glue on a button)
Cut a hole in it so you can button it up.
Now, to stop this from fraying, coat the edges with clear nail polish.
Now, clean up any unfinished bits, sew on buttons, lace or ribbons, splash paint over it, put bird poo on it, throw it off the Effiel Tower-whatever you choose to do.
Just button up your bag-and go!