This is something I've been dabbling with for about a year now. I draw the image on a t shirt using a squeeze bottle filled with screenprinting ink, and then use a piece of cardboard to move it around to create grays and patina. The ink is a water based textile ink which softens a bit after being washed, so the image has a sort of "lived in" look.
They're fun to do, as I get to be a bit messy, and I can draw the same thing multiple times and never wind up with two identical images. For instance, the next time I draw Truck Monster, maybe there's someone behind the wheel, or perhaps I chop the truck. They also finish quickly, as it's supremely difficult to be fiddly when you're essentially drawing with a glue bottle and a spatula. You have to go with the flow, which can make for some interesting results.
I've put a few of these drawn shirts up on ebay. The shirts are all Hanes Beefy T in classic white, different sizes. Quantities are, naturally, limited, though I do plan to add more designs as time marches on.