I always cut from the bottom side, with circular saw, 40th carbide trim blade. Set the blade about 1/2 past finish surface. Minimizing the depth puts the teeth at a more optimum angle as it passes the surface, resulting in better finish cut. Do layout on top, transfer to back side, clamp on saw guide. The hardest part can be cutting the backsplash if it's a one piece top. It's doable, but definitely needs a high skill level to accomplish. Grinders, belt sanders. & block planes all help to do adjustments. Do wood glue on joint, with silicone at the finish surface to help seal out water.
Joe