Also, check out the accompanying YouTube video:
You start by using a piece of plywood with one good edge. Either the factory edge that was cut when you bought it, or one you cut with your table saw. If you don’t want to make marks on the plywood, tape a piece of paper to it, and mark on the paper. Lay the short edge of the square along the plywood, draw a mark the entire length of the square. Flip the square over, and move it about 1/16″ of an inch away from the other line, and draw a second line. These lines should be parallel. You will see from the video above, that for a framing square, you can use a center punch and if your square is less than 90 degrees, hit the inside corner, if your square is more than 90 degrees, hit the outside corner with the center punch (hard), on a metal vise. This spreads the metal out ever so slightly and will move the angle of your square slightly. Be sure and check it again on the plywood, again making two marks with the pencil. See if you need to do it again. You can make several punches in a small area to spread the metal more and more if needed. Make sure and don’t go too far. Also don’t get too close to the edge, or you may mess up the straight edge, and you’d need to file that down.