Cut across the top of the shirt, just under the sleeves

Cut down the side of the tube and turn it so that the bottom edge of the shirt is now the top.
Decide whether you want to use the outside or the inside of the shirt as the outside of the bag; some shirts have a smooth side and a slightly rougher side- depends on your preference.
Lay it flat and get out your cassette and paint.

Paint the cassette, then press it onto the shirt like a stamp. Do this in various directions and colors as desired. Let it dry.

If you want to add a lining, sew it on then fold the whole thing in half, inside facing out. decide how wide you want the bag to be and mark and pin it. Sew up all the sides except the top.

Leave the about and inch and a half un-sewn on the top for the straps.

For the casing, you can do it the regular way and make the casing, then string the straps through, or you can just fold and pin the fabric down over the straps and then sew it. Either way you do it, make sure that you put the strings in right.
<---
When you pull the strings, the bag should close. Don't mind how low i put the strings in the picture, that's just for reference :P

Turn the bag back right side out. Sew the string ends to the bottom corners of the back of the bag.

Done ! (: