Off set it based on the baseplate of your circular saw. For longer cuts on plywood use a metal straightedge and a pair of clamps to hold the straight edge in place which you can then use as a guide for the circular saw in the same manner. Locate the front of the circular saw.
Just a tapered extension on one side or a guide at an angle to ride the miter slot.
Support the board with your left hand. Hold the board snug against the fence and slowly push it through the blade. Now mark the exact length at the other end and align that mark with the end of the extension fence Photo 1. Theyre rarely perfectly square.