Originally Posted by amma

I square up my blocks, then my rows, and then the top again before adding borders, after each border and then the whole top.
Hopefully doing this as I go, only a little bit needs to be shaved off here and there, rather than a 1/4" or more here and there.

A 12" square up ruler laid on the corners, then line up another ruler or two or three across the top.
Nice straight yard sticks are helpful too. The metal Tsquares you can get at a hardware store are also a good tool to use.

You can also lay your quilt out on the floor tape or pin it down. Tape or pin a string onto the carpet/flooring on either side of the top. Use the string as a straight edge guide, and slide your rotary mat underneath the quilt and trim as needed.