Well, Kirsten, you certainly can sew the seam on the long edge of the triangle after cutting the square in half diagonally. However, then you are sewing a bias seam which easily stretches. I have had the most luck with a few other techniques: Cutting the square a bit larger, doing the diagonal line with stitching 1/4 inch on either side as you describe, cutting between the stitching, opening to the square, pressing carefully (usually to the darker side), and trimming the resulting squares to size. I also really like using paper like "Thangles" or whatever brand has the squares with the stitching lines printed - usually comes in sheets that can be cut to as many squares as you need. With either of these methods, one pins the darker and lighter pairs of fabrics to the paper on which you stitch.I run my stitching just inside the printed line to be sure the square is at least big enough and not skimpy - the 1/4 inch is not as important as the size of the square when finished! Then cut between the stitching which gives about 1/4 inch seam allowances. Again, although the paper pattern is very accurate, I check the resulting squares and trim them if necessary.
kendzis :D