To cut a post-formed countertop, place the piece upside down on a pair of sawhorses, attach a plywood guide strip along the line you want to cut, and saw along the line with the fine-toothed blade of a circular saw. Smooth the cut edge with 220-grit sandpaper.
Measure the countertop and surrounding kitchen appliances to decide how much material to remove, and draw a line on the underside of the countertop to mark the cut. Place rags or other cushioning materials atop the two sawhorses before placing the countertop face down on them. Cut a 3/8-inch piece of plywood to make a guide strip, clamping it along the drawn line to facilitate cutting. Use a circular saw with a fine-toothed blade for standard post-formed countertops, or use a jigsaw if the piece has an attached backsplash. Always use protective eyewear when sawing.
For an even more precise cut, create a 3/4-inch plywood base to attach to the underside of the countertop underneath the guide strip. Use biscuits and screws to assemble the base and attach it to the counter. Re-trace the cut line, and screw or clamp the guide strip atop the base. Use a circular saw to cut through both the wood and the countertop. Remove the plywood, and sand the rough edges before installing the countertop.