Before you start, find a pair of tights that you are willing to give up, and flatten or iron them so they aren't wrinkled. And they would be more workable too!
Once you're ready, cut the tights at around the ankle area. (It depends if you want it long sleeve or short. If you want it shorter, go for the knee area and continue from there.)
This is where you should be right now. #1 TIGHTS #2 SCRAP
Now, cut the crotch panel along the seam. I would suggest using a pair of tights with the panel if you are doing this for the first time. I have a video of the cutting process on page 6.
It's hard to get the sizing right since it seems like your head can't fit through that small hole (crotch panel) ... But trust me, the hole stretches once you cut it!
Once you are done cutting you are basically finished! Although you could always decorate your shirt.
Finished Product! Thanks!