Beginning with a layout I've designed in photoshop, I use my lightbox to get a pencil outline.
I then use the lightbox again to draw my outline in marker.
Once I have a solid outline to work with, I fill in the necessary shapes.
After the drawing is complete I take it into the computer and make the shapes vector. This is the step I pull out all of the corners to make them rounded to produce a worn/stamp look.
Here's a look at the graphic on a T.