You've ordered your granite countertop and purchased your undermount sink. The opening for the sink has been cut in the granite and the cut edges finished and polished. With focus and confident determination, mounting a sink to the underside of a granite countertop is pretty straightforward. The important thing to remember is not to rush the job.
Crawl under the granite countertop where the sink is to be mounted and check the granite for dust. There may be dried-on grit and dust deposited on the underside of the granite from when the sink hole was cut and the edges of the hole polished. Brush and wipe away any dirt and grit.
Apply silicone caulk to the top lip of the undermount sink and position it on the floor of the cabinet right under the cut-out hole in the granite countertop. Push the sink up so the silicone comes in contact with the bottom of the granite countertop. Make sure the sink is centred beneath the hole.
Run one end of a pipe clamp through the drain hole of the sink. Make sure one of the clamping jaws is positioned underneath the sink. Lay a length of 2x4 lumber across the sink opening on top of the countertop. The 2x4 must be longer than the sink opening. Rest the second clamping jaw on top of the 2x4 and tighten the clamp until the lip of the sink makes contact with granite. Check again to make sure the sink is centred under the hole in the granite before tightening the clamp firmly.
Mix up the two-part epoxy per the manufacturer's directions that came with the undermount sink. While lying under the sink, apply a dab of epoxy to the flat end of a bolt post and press it to the underside of the granite next to the lip of the undermount sink. Repeat for every bolt post until they are evenly spaced all around the perimeter of the sink.
Screw the rest of the metal mount mechanism to each of the mount posts after the quick-setting epoxy has set up--it usually sets in 10 minutes. Tighten the metal mounts until they make firm contact with the underside of the sink lip.
Allow a full 24 hours for the epoxy to cure before removing the pipe clamp and installing plumbing in the sink.
If you are installing a double sink, you will have to use two pipe clamps.
Don't rush Step 6, as it's the epoxy glue, with an assist from the silicone caulk, that will be holding the total weight of the disposer while the sink is filled with water and dishes.