I use my husband's large metal T square and start in with the upper right corner to make sure that is square both ways. Usually I only have to do a very slight trim or none at all because I do square up my blocks as I go. Then I switch to my 6x36" ruler and using the outer edge of my 2nd to last border I line up my ruler to trim the outside border a slight bit, sometimes up to 1/4" to get rid of variations in the outside edge usually due to the machine quilting pulling a little at the edges. Now I just keep his T square in my sewing lab and he knows where to find it if he needs it.