Hope I can explain this so it makes sense. If your square is rather small take an index card and lay it diagonally from corner to corner, line your machine foot edge up against the index card and adjust your needle to 1/4" to the side of the index card and stitch. No need to mark anything. Flip it over and sew down the other side in the same manner. if your squares of fabric are larger, use a file folder or larger card. This is not my idea, a quilt group member shared this recently.
I've also used this method ... because I'm lazy and it makes several at once and no stitching on bias ... hope this makes sense ... if the square in your picture is say 2 1/2", cut 2 5" blocks (1 of each fabric) and on one piece (I do it on the lighter fabric) mark the centers on all sides and draw lines on center top to center bottom and center side to center side (your block now has 4 sections) then draw a line diagonal from the the center mark on top/bottom to the center mark on the sides. These will be your cut lines after you stitch. You can either draw lines or use your foot to stitch 1/4 inch on each side of the drawn diagonal line. After stitching, cut and you will have 8 1/2 square triangles. Again, not my idea ... read it somewhere.
Suzy