ok here goes....
i tried to do this w/out actually cutting up my tee cuz i have cut up endless amounts of sleepshirts exerementing and have about a zillion in this cut up style...but if this is still confusing just let me know and i'll steal one of my brothers undershirts and actually do it for ya!
mine is tied in the lower back because it was way too big, but if u have a shirt that fits right already you can leave that step out. i didnt explain how to do it here but lemme know if u need to
cut on the black lines...the red arrows are showing that you need to continue cutting to the back of the shirt.
its easier to do the horizontal cut along the back of the shirt if you first cut off the sleeves of the tee at the seam, so it looks like a muscle shirt. then, line up the side seams of the shirt and just cut straight across. the piece is still attached to the shirt from the neck area tho. use that to make the tie around ur neck.
well, i hope i made sense...im not sure. i dont think i did a very good job of explaining w/ out real pictures. u might wanna try it on a shirt you dont care about first to make sure that my instructions were correct. You also have to make sure that the shirt you are using wont fray after you cut it. shirts they give away at parties, school, and charity stuff are what i use. I used shirts from camp, Relay for Life, Walk for the cure, and dance.
hope i helped! feel free to speak up if something doesnt make sense or u need help. Good luck!!