Once you have your shirt, lay it out and cut off the sleeves, neck, and bottom. You should have two squares(back and front of the shirt) of fabric a bit bigger than the final bag.
Sewing the bottoms together
Take the squares and lay them on top of each other(pattern sides together), pin the bottoms together, and sew ONLY the bottoms - the other sides will be sewn later.
You may want to re-sew it a few times for extra strength.
Hem the tops
On the patterned square, take the top, fold it down, and pin it, so that from the outside it looks like a normal bag(no jagged, cut edges). Make sure that it is straight. Then sew.
Repeat with other square, lining the hem up to match it matches the first one. (Matching the two isn't necessary, but it makes the final product look nicer)
Sew the sides together
Fold down the squares so that they are lying the way they were when you sewed the bottoms together(pattern sides together). Pin the sides, and then sew them. (Once again, re-sewing can only help)
Making the strap
Take a bandana, and fold it into a triangle(the way you would if you were going to wear it on your head). Then fold it over and over into a strap until you get the desired width. There will probably be a little flap of fabric in the middle of the strap. Sew this down, and then the strap is ready.
Sew the strap onto the bag
Pin each end of the strap along the side seams of the bag, and then sew. (Yet again, re-sew a couple of times for strength.)
And enjoy your new bag!!
Feel free to email me with any questions concerning this project at firstname.lastname@example.org
Have a nice day!