3rd pic... Once the measuring for square is done and checked twice and the sawboard is secured, I make the cut. I carefully set my blade depth to not quite cut all the way through the face of the plywood. I'll clean up this edge after the fact.
4th pic... the cut is made! and the bad stuff is removed. The original cabinet had dados (slots) cut in it to align the parts. I'll be re-using the "tabs" on the bottom, front and back of the cabinet to strengthen the glue-up of the new piece.
5th pic... closeup showing the bit of fibers left after removing the bad parts. Also shows the factory tongue that will stick into the grooves that I will be putting in the back of the patch. On a williams cabinet you also have to dig out the spring steel "nails" that they used to hold stuff together. These are hell on a saw blade, so if you do this repair, you might want to have a spare blade on hand.