first, decide what, and how many organs you want to put on your shirt, i used the heart, lungs, liver, stomach and small and large intestines

after you have chosen your organs, google them and sketch them up on some type of paper. (these will be come your outline stencil) doing this on paper first allows you to play with the shape and size a bit before putting it on the tee so you can make sure your result is what you anticipated.

then once you are happy with them, cut them out.

next, arrange your cut out organs onto the shirt into the arrangement of how you would like the final product to look.

then i took a pencil, and starting at the bottome of the shirt, started outlining my organs. i outlined what you could see of the intestines,then removed the shape, then the stomach, then the liver, then lungs, then heart. after i had the basic outlines, i added the details to the heart, and lungs, and small intestine.
(sorry for the bad picture, my pencil outline was dark enough to help me paint, but not dark enough for a good picture)

after i had the pencil outline, i outlined it in black paint.

*don't forget to put something inbetween the shirt's front and back so the paint wont bleed. i used a large construction paper pack, and flipped it over so that the sturdy cardboard was what was against the side i was painting.

then determine the colors you wish to paint your organs and start filling them in like paint by numbers. if you have a good paint consistancy and a steady hand, try not to paint over the lines, or just paint like a retard like me, and plan on going back to touch them up.

when your done painting & the paint dries, it should look like your own version of this...
and then you have an awesome science geek shirt!!!!!! :]

i like the outcome of this so well, i plan on doing and axial skeleton one, and maybe just a digestive system one(i thought itd be fun to paint in a pizza or half aten cookie going through the digestive system track)