Measure up, mark your line(s). https://poal.co/static/images/Cs8VYo.jpg Position your saw so that the blade lines up with your line, then line your guide up with the saw. Square up the block and clamp it down. https://poal.co/static/images/ek2NvZ.jpg Using that scrap wood guide for the circular saw's guard, you will make a perfectly flat and square cut. https://poal.co/static/images/kwAtjC.jpg