Basically the author adds on a thin strip of wood that is cut off roughly where the blade would start cutting on a circular saw, This way you can line it up and then run the saw along side the edge of the square on a plank to get straight cuts. You can get speed squares pretty cheap so this could a cheap jig to make too.
https://www.homemadetools.net/forum/speed-square-jig-circular-saw-cuts-81001?homelistings#post164513