I have a large square ruler. These are great for squaring blocks.
Here is how I square my blocks:
Example: block to be 9inch finished block should be 9.5 square
Find center of block Around the 4 3/4 mark
Find 4 3/4 point on ruler, place that over 4 3/4 on block You will have two side exposed, cut those to ruler
Turn block 1/2 around and repeat.
If I am being super precise I do put a little mark on the block for center. I fold in half lengthwise then crosswise.
Hope this makes sense, if not let me know and I'll explain it more in detail.
If you square your blocks as you construct the quilt, your border will go on much easier and the quilt will lay much nicer in the end.