Cut the substrate down so that it's slightly smaller than the cabinetry as recommended in the instructions from your stone supplier. Mark the area of the substrate that needs to be trimmed off. Then align the edges and use the circular saw to cut it to size.
Place the substrate into position and screw it to the bottom cabinetry. Always attach up from the underside. This means if you ever want to remove the benchtop, you can still access the screws and fixings. Check the length of the screws to make sure they won't poke through the substrate. Make sure the substrate is level because stone only has a 2mm tolerance.