Sorry, it was from an old magazine, but the general idea is
1. Sew strips together (say 6") to any length you want
2. Take any square ruler and cut out 1/2 square triangles (vertical) and sew 4 of them together to make a square. The larger the ruler, the larger the square.
3. Cut a piece of plain material the same size as the string square and sew together
I hope this helps
Sharon