I would cut 3 strips of 2 1/2" from both fabrics, sew together in alternating colours. Then cross-cut into 2 1/2" units, you will have say red on one end and white on the other. Join these units, every 2nd row will start with a white square, and the 1st and alternate rows will begin with the red. You will have a 12 1/2" block.