This seems simple but I was having a very hard time squaring my blocks.
In this picture I am trimming the right side. I placed the small ruler so it extends 1/4" to the right of the center of the square. Then I placed the 6" wide ruler next to it, and I can trim the whole length of the right side. I made sure the rulers are lined up with the straight edge of the bottom of the block.
I already did the top and bottom this way. In this pattern it was easy to determine where the center of the block is. If it is not visible you can simply fold the block in half to leave a crease in the center to work with.
I hope I explained it well.