Empty the sink cabinet and position a towel or bucket inside to catch drips. Turn off the water supply under the sink or at your home's main supply lines. Turn on the water at the sink to empty the lines and to make sure the water is off. Disconnect the stopper lever first; twist to loosen the locknut that secures the trap to the sink drain. Use an adjustable wrench to loosen and disconnect the hot and cold water supply lines.
Remove the sink
Look up inside the cabinet to examine the bottom side of your old sink and see if you can locate clips around the rim that help snug the sink to the countertop. (Metal sinks usually include these clips while porcelain sinks do not.) Loosen the screws holding the clips in place (use a screwdriver, if needed) and remove them. Now, working from above, slip a box cutter blade or a razor blade between the sink rim and the countertop and cut through the caulk sealing the sink to the surface. Be careful to not damage your countertop while cutting. Lift the old sink out of the countertop, and use the razor blade to remove any excess caulk remaining on the countertop surface.
Prepare the new opening
Measure the existing hole in the countertop. Select a sink that fits the existing opening or choose one slightly larger.
Lay the template that comes with your new sink over the existing opening. If the template is larger than the opening, draw around the template to create a cutting line. Cut out the outline with a jigsaw to enlarge the opening to fit your new sink.
Prepare the new sink
Following the manufacturer's instructions, install the faucet on the sink as well as the drain and stopper pieces. Run a bead of caulk or plumber's putty around the underside of the sink rim.
Set the sink into the countertop opening, aligning the sink drain stem with the trap; run plumber's tape around the trap threads for a watertight seal and reconnect the trap locknut as well as the stopper lever. For a watertight seal, run plumber's tape around the threads of the water supply line connectors, and reconnect the water supply lines to the appropriate hot and cold connections. Tighten screws to sink clips, if any, beneath the sink. Run a bead of caulk around the outside edge of the sink rim and smooth. Turn on the water supply. Turn on the faucet and let the water run for a few moments to check for leaks.