Spread your shirt on a flat surface

start drawing your skull, for help i went on google images to see what other shirts like this looked like

leave a line in the middle of the eyes to make it look a bit better
There is two lines at the side because i wanted the eye bigger

then make the nose, make two tear shapes on either side at the bottom of the eyes in the centre

for the teeth try to raw a straight line in the midle and draw triangle shapes for the teeth

cut it all out

stretch to give a better effect

the eyes

eyes an nose

top row on teeth

all of it done

stertch the cuts you just made for a better effect

cut the sleeves if you need to in a circle shape and done