Think of "rows" on the diagonal, not up or down, or side to side. The first row is from the top left corner, diagonally to the other side, long enough so that the quilt is wide enough. The second row can be below the first; then it will be the same length as the first row. Continue this length of rows until the quilt is long enough. Now each row will be 2 squares less. When the corner is reached, finish the top right corner in the same manner. Do not sew the rows together yet. Make your setting triangles and corner triangles and sew them onto the correct rows. Then sew the rows together.
I make the setting triangles a little larger so that they have a little float and can be trimmed to about 1/2 inch outside the points of the squares.