Soak the shirt in soda ash water until it's completely saturated. Squeeze out the excess. Crumple it into a small plastic container, freeze the thing solid. When it's frozen, apply the dye to the top of the ice cube of shirt. As the shirt thaws, you get the crackle areas that the dye travels through. When you think you have enough color (and this is the tricky part), rinse in cold water until the soda ash is gone, then very hot to remove any excess unreacted dye particles.