This is what you'll need:
Step 1: Lay shirt on flat surface and cut a straight line just below the armpits.
Step 2: Measure two-fingers-width at the top of the cut t-shirt. Cut that width all the way across your t-shirt. This will become your thread - Set aside.
Step 3: Turn your t-shirt upside down, so that the edge you just cut is facing you. Cut "piano keys" all along the bottom, creating fringe.
Step 4: Cut a diagonal line (or triangle) in the hem of your t-shirt on both sides
Cut your thread to make one long piece. Attach your safety pin to one end of the thread.
Or if you don't fancy the fringe, turn it inside out for a hobo effect: