To install a granite countertop, prep the counter, cut the sink hole, join the seams, and then glue the countertop down. The last step is to glue the seams together.
Cut a 3/4-inch thick piece of plywood to fit on top of the cabinets, but make sure it does not overhang. Attach the plywood with screws, making sure to center them on the frame underneath. Place the pieces of granite on top of the plywood to ensure that it fits correctly. Trace the sink hole onto the plywood subcounter, and then remove the slab of granite. Use a jigsaw to cut the sink hole out of the plywood, making sure to cut about 1/8-inch larger than the opening, and then drop in the sink.
Next, level the seams for the slabs by using screws to adjust the height of the slab. Use a level to ensure that all of the slabs are even. When all slabs are even, lift the granite, and put dollops of silicone around the cabinets every six to 12 inches. Place caulk around the rim of the sink, and then apply caulk around the top of the rim of the sink before lowering it down. The last step is to glue the seams together with putty and hardener.