Take ur shirt and cut through both layers. cutting it into a rectangular square shape
take your double layered squarish rectangle and needle and thread then with the square inside out, sew around all sides except the opening!!
take the opening and make a narrow hem by folding about a half inch of fabric down and pinning it in place all around. Then take a zipper and pin that over your new hem and sew carefully in place. Use dental floss so it will be extremely durable. And minty fresh.