Measuring with the ruler, cut rectangles from the jeans in the following sizes: two pieces measuring 8 inches by 12 inches for the front and back; two pieces measuring 4 inches by 12 inches for the two sides; one piece measuring 4 inches by 8 inches for the bottom.
Lay your rectangles on the table with the 4 by 8 piece in the middle, the 8 by 12 pieces on the top and bottom, and the 4 by 12 pieces on the sides.
Grab a parent and use the hot-glue gun to put a line of glue along the short edge of the front piece on the outside part of the jeans.
Press the glue edge to the outside part of the bottom piece, carefully aligning the edges.
Repeat steps 3 and 4 with all pieces along the bottom, then glue the side pieces to the front and back pieces.
Use more glue to seal any gaps along the bottom and edges. You should now have a sealed bag with four sides and a bottom. Next cut two strips of fabric from the jeans, each measuring about 6 inches long and 1 inch wide.
Place a small dab of glue in the center of the front panel of the bag, about three inches from the top.
Press one strip at this point, facing up and down. Repeat on the opposite side of the bag with the other strip.
Decorate the outside of your lunch bag. Then to close it, roll the top of the bag down and tie the strips together.
Pack a reusable water bottle and metal utensils in your new lunch bag.