For this project you will need a pair of old children's overalls from a thrift store. The larger the overalls the bigger you bag will be.
What you need
If your overalls has any pockets on the front it will make a great little mobile pocket on the outside of your bag.
Start by cutting the legs off, making sure to cut above the crotch or you will end up with a tuft.
Turn the bag inside out and using an overlocker or a sewing machine sew across the bottom to close it up. Make sure you do 2 zig zag stitches on a sewing machine to strengthen it.
Trim off any threads and turn the right way out.