I think what she is saying is the smaller block is the square, that stays square. Then the larger block is cut on the diagonal into the triangles and sewn onto the sides to make the finished triangles for the sides. I couldn't say for sure, but, that is kinda what it looks like from the picture.
Edited to say ... Remember to match up you straight sides, then there wil be a dog ear hanging off in the middle. It is supposed to be like that, you can trim it off after you sew the other side on and press it so your seam is straight along that side. Press up and down ... no rubbing the iron around back and forth! Just put the ruler on the block and make sure you leave a full quarter inch seam. Just the dog ears, really need to be cut.