I have been trying to square off some 12" blocks. I have a 6.5" square ruler and cant work out how to do it( it's the only square ruler I have )

I consider myself ok at most aspects of p/q but for the life of me I have never got the hang of squaring blocks or cutting straight

@Iminei gave me the above mentioned ruler, complete with sandpaper dots on to stop it moving....but.....they don't!!

I ruined a whole fat eighth because it slipped

I cannot stand, bent over, for very long and often cut sitting down which doesn't help

I think it'll be pre cuts from now on...but.....his doesn't help with squaring.........

**give me a foolproof method** please