Step 1: Test for squareness
Butt the square up against your straight edge and strike a line. Flip the square over and strike another line starting in the same place as the first. If your two lines diverge then your square is not square.
Step 2: Adjusting the square
To adjust the squareness we adjust the height of one of these islands by wrapping the blade of the square in sand paper and grinding it down. Which island you target depends on which direction the blade of the square needs to move.
Only grind a little bit at a time and test for squareness frequently. A little grinding goes a long way and it's really hard to put the material back on once you've removed it.