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
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