The method you are describing is snowballing a square. In order to draw an accurate line, get a piece of fine sandpaper (220 or higher). cut the square, place it on the sandpaper & draw the diagonal line. The sand paper keeps the square from stretching. Use the smallest marking instrument (I use a sharpened pencil). Place the small square on the larger square and sew right next to the line towards the outside corner, practice does make perfect. Another trick is when you fold your corner over is it not necessary to cut the corner off the large square. Leaving that in place gives you the perfect sewing line when joining it to the next piece. just cut the excess from the smaller square.