After you have soaked and wrung out the shirt (by the way if you don't do it in the washing machine, try wringing it out with a friend!)place the shirt on a flat surface. Place it right side down, as you will then get a sharper pattern on the front of the shirt. Of course if you want the sharper pattern on the back - well, you know!
Place the dowel rod, (or your finger, or a wooden clothes pin) in the centre of the shirt and start turning clockwise until you have a nice flat pie shape. You can also turn counter-clockwise if you prefer, it makes no difference really.