Draw the shape you want on the back side of the fabric. Turn the shirt inside out and pin the shape (backside up) to the shirt in the position you want. Sew along the line (sewing only the fabric and one side of the shirt).
Turn the shirt right side out. You will be able to see the sewn line. Carefully cut the shape out of the t-shirt, being careful not to cut the fabric layer. Don't get too close to the stitch line, leave 1/4 inch of t-shirt.