Do it like flying geese. Cut a white rectangle the size you need, and cut blue rectangles to fit. So, for example, if your border pieces are 6x4 finished , then you would cut 6.5 x 4.5" white rectangles, and 2 blue rectangles 3.5x4.5". Lay a blue rectangle on the white, against one end, and sew along the diagonal. Cut, flip, press just like a flying goose. Then do the same with the other end.