When designing a drawing for a custom T-Shirt, think about where you want the center of the design. Think about the size of the design and the amount of letters to be printed. For instance putting in the o2 logo would be less complicated than putting in a full http://www.o2.co.uk/ URL.

If you want the center to be the center, then start in that location. If you want a more offset design, then offset the design. The image will be printed on the shirt using the center of the drawing as reference to the center of the shirt.