Take a piece of matt finish sticky tape and attach it diagonally to the front side of your 12 1/2 inch square ruler. Begin at the centre of the top edge to the centre on the right edge.
 
Similarly place a second piece of tape from the right edge down to the bottom edge.
 
Using a permanent marker pen, draw a line from the 6 1/4 inch point on the top edge to the 6 1/4 inch point on the right edge, beginning and ending 1/4 inch in from the edge.
 
Draw a second line from this point down to the 6 1/4 inch point on the bottom edge.The two lines will create a perfect right angle.
 
square it up
 
After stitching the triangles to each side of the square, place the ruler over the work so that the marked lines lie over two of the stitched lines.
 
There will be a 1/4 inch seam allowance between the point on your block and the edge of the ruler.
 
Trim the edge. Turn the work through 90 degrees and repeat the placing of your ruler and the cutting.
 
Repeat with both remaining edges.
 
You now have a perfect square in a square.
 
square it up