Literally any ruler can be a square up ruler. I have several I use all the time. As long as you understand the basics of squaring up, you will be fine with whatever square you have. The website gives the method I use, except that I never pull or push the fabrics to make them be square. I find they always want to go back to what they were before. I will rip the block apart and make some smaller seams if the block is too wonky.
http://justquilty.com/2012/08/how-to...sewing-series/