I would do 1 row of 2 blocks at a time. First add sashing to the smaller block to make it the same size as the larger block in the row. You can add to the top, to the bottom, or both depending where you want the smaller block to lay. Then figure how wide a sashing between the blocks to make your 24 inches. I would do this for each row, then figure how much sashing between the rows to make the length you want. Kind of a one-step-at-a-time method.