STEP 1: Draw a diagonal line from one point to the other on each of the white squares. Use whatever fine point marking tool you have on hand. This mark will eventually be in the seam allowance, so it does not need to me removeable.
STEP 2: Matching each white square with a colored square, right sides together, sew 1/4 inch from each side of the marked line. If you are using a striped fabric, I recommend laying the striped fabric so that the marked line runs perpendicular to the stripes. Whatever you choose, make sure you are consistent, so your stripes all run the same way.
TIP: I like to sew mine in a chain. Saves a lot of time!
STEP 4: Cut along the marked diagonal line on each block. Do your best, but do not worry if you don't follow the lines exactly, as this will be the seam allowance. Press all of the seams to the colored fabric. Also cut off the dogears on the corners, where the seam overhangs. In the photo below, I haven't trimmed them yet. This will really help to reduce bulk.
STEP 5: Assemble the block. Layout each block as seen in the photo above. Begin by sewing the top two blocks together, and then the bottom two. Do not forget to press your seams. I like to do them to the dark color, especially when using a white fabric. Next you will sew the top row to the bottom. Finish off your block by pressing.