Welcome to the board from Southern California.
When I square up the quilt top I use a 12 1/2" large square ruler in the corners and try to get a good square corner while making sure that I am not making the boarders real uneven to do so. I then use a 6 x 24 ruler to trim up all the sides trying to keep the borders a consistent width.
Some of the folks here use a carpenters T square.
I hope this helps!