You need to start stitching a 1/2" from the center because the exact center is where you are going to tie your ribbon or yarn into a bow.
Pinch up about an 1/8" of fabric with your fingers. You need just enough fabric for your ribbon or yarn to thread through. Start sewing along the bottom of this pinched portion so that you are creating a tube. After a few stitches, test to see if your safety pin will fit in the tube. The safety pin needs to fit because you'll use it to thread your drawstring through the tube.
Sew all around the shirt, being careful not to bunch up the shirt with your stitches. Stop when you reach the front again, 1/2" to the right of center.
Tie off your thread. Now it's time to clip holes for the drawstring.