Use the Tape measure and measure in from each of the side factory edges 12". Do it on both sides and both ends. I marked it using a sharpie so it would show up in the pictures. Next set it on top of the 2 2x4's leaveing a gap of 3-4 inches.
Set your Circular saw so that it just cuts through your ply and does not cut into your work surface.
Run it down both lines so that you have two strips 12 inches wide and 1 piece that is about 23 1/2 inches wide (You wil need to measure yours for the next step).