I begin with one large square (this one was 9 1/2 inch) and four small squares (these were 4 inch). The four small squares are marked with a diagonal line from tip to tip as you can see in this picture:
Of course, you only sew them on one at at time. So take one small square and match its unmarked corner to the corner of the large square. You may want to pin it in place; I don't but I'm a bit careless about things like that.
The diagonal line (which now runs across the corner of the large square) is your seam guide. Run you quarter inch foot along the line, so that your seam is a quarter inch from that line. Do that on BOTH sides of the line.