Step 1: Purloin your boring old shirt
Step 2: You are now spoiled for choice...
Choice One: You can pretend you are doing a rainbow swirl tie-dye http://www.instructables.com/id/How-to-tie-dye-an-old-white-shirt-or-a-new-shirt-/ and place the shirt face down, place a dowel rod in the center and twirl it anti or clockwise to achieve "pie", and put rubber bands on to hold its shape
Step 3: Second choice
You can scrunch up your shirt in an irregular pattern and then keep in shape with rubber bands back and front.