This is a project from 99 Ways to Cut, Sew, Trim, and Tie Your T-Shirt into Something Special by Faith Blakeney. If you want to re-design a boring t-shirt this is a great way to do it.
Lay out t-shirt flat. Cut along the bottom edge of the shirt so that it is rounded out at the edges (in the book this is called a hipster style).
Also, cut the neck out of the shirt starting about 1 and a half inches from the shoulder seam. (See this photo for an example of what the top and bottom look like).
Flip the shirt over. Cut out a square out of the back of the shirt starting just below your shoulder blades. With square you cut out of the t-shirt cut into swirl starting on the edge and working your way into the middle of the shirt. It will end up as a long t-shirt string.
Cut 4 slits on the sides of the cut out square on the back of the shirt. Take the t-shirt string and thread through holes on the sides of shirt, tie at the bottom. And, then you have your new sexy shirt :)