Position the cutting template included with the sink in the desired location on top of the counter.
Draw a cutout line around the edges of the cutting template with a pencil.
Support the underside of the portion of the countertop you intend to cut. As the counter is cut, the weight of the cut portion will likely cause the opposite end of the cut area to break off. Supporting the counter from below will prevent this from occurring.
Drill a starter hole just inside the cutout line that is large enough to accommodate the blade of a jigsaw.
Insert the jigsaw blade into the starter hole, then turn on the saw and cut the countertop by carefully following the cutout line.