Originally Posted by QuiltingGrannie
I usually lay the block on my cutting mat at 0 (zero) and whatever size needed, which in your case is 12 inches, and use the square corner of the ruler to make sure it's square and trim off the excess. Be sure to press the block before you trim it square.
I think this is what I have to do. The block size is 12" according to the pattern. It will be 11 3/4" after the blocks are joined. This seemed odd to me, but it's the first Log Cabin I've done. I thought I'd be able to easily find a 12" square ruler to lay on top of the finished block-centering the "center" red square and trimming the edges.