It started out with my friend and I wanting to make our own shirts for Warped Tour this summer, so we were on a search. I saw chelsearaye's anatomy tank and I loved it, so I decided to do my own, but make it a little more "graphic." The only reason this is "so-so difficulty" is because it depends on how much detail you want to put into it.
Also I am just guessing about the time. As stated before, the more detail you put into it, it will most likely take you longer.
Pick out a few pictures of the internal organs of your choice for your shirt.
I googled the organs I wanted, and then sketched them how I wanted on a piece of paper.
On the shirt you picked out, you can do one of two things.
1. You can free hand it based off of your sketch.
2. Draw the organs seperately on sheets of paper and use them as stencils.
Once you've either free handed or stenciled the organs on, you can start to paint.
I personally recomend starting out painting the insides first, no pun intended, and then outline them with black.
To be safe I waited about 2 days to let mine dry, and then added a redish black mixture around the edges to finish it off.
Once it's dry, ENJOY! :)