This is a simple task, but if not paying attention can get tricky. Place the light square on top of the darker square because you have marked the diagonal line for your cutting line. Place your pressure foot along side the cutting line and sew a 1/4 inch seam. I chain stitch and at the end of the pieces you are stitching turn it around and stitch the other direction.
As you can see the chain stitching works well and you do save thread with doing it this way instead of cutting each set as you stitch.
When cutting the pieces apart you will have a square with two lines of stitching and now the fun part cut on the drawn line and you have two HST pieces to work with. As I said easy. Hope this takes some of the scary part out of doing it the old fashion way. Chris