I've made a card like that for my brother using a template off this site:
http://members.tripod.com/cecilia_liu/shirt.htm
It's for origami, but I just used the finishing photo as a template. I folded my cardstock in half and drew the shape by hand. Just make sure that the top of the shirt is at the fold. Then, I trimmed around it so that the hem of the shirt will flip up and open as a card. I decorated it with a pocket and buttons. I hope that helps.