1. Make sure the hem of your shirt will accommodate a large safety pin or plastic needle and the rope you intend to use. When cutting a small slit in the hem to test this, cut it in between the side seams. The ends of the rope will need to come out in the middle, so the side seam of the shirt fall to each side, when completed. Don't feed the rope through yet, just make sure the hem is large enough.
2. Turn the shirt inside-out, and sew a line (to close it) from right under the arm seam to the other side. This is going to be the bottom.
Insert the pin into the slit that was cut on the hem of the shirt, and work it through the entire hem to come out on the other side, gently pulling the fabric around the rope.