Place a bead of silicone caulk all the way around your countertop where the granite sink will be installed. This is needed to ensure the sink won't leak.
Lift the granite sink and set it within the countertop hole. Make sure the top edge of the sink is set on top of the silicone caulk bead.
Put together the brackets that came with the granite sink. One side will attach to the sink and the other to the countertop.
Slip the brackets in place on either side of the sink from underneath the counter. The sink side will slide into place.
Put a screw through the other side of the brackets, and screw it directly into the countertop to secure the brackets and the sink.
Wait for the caulk to completely dry before you use the granite sink.