Sew the arm on the marked position with slip stitch around. Knot the thread and insert the needle to the other side of the arm position. Pull the thread so that arm “squeezes” into the body, knot to secure the shape. Sew another arm with slip stitch. If you think that the arms are not “sunken” enough, pass the needle to the other side, pull the thread to get the result you want, knot the thread securely.