Soak your shoes in water so they're pretty waterlogged and use water based magic markers to draw solid shapes of different sizes and colors onto the wet canvas. The inky shapes will bleed around the wet fabric and overlap in spots, giving the illusion of tie dye. (Think of what happens when you drop food coloring into water--it's the same effect). Let the shoes dry completely, then spray with a sealant such as artist's fixative or clear spray paint.
