Before you begin, shut off water to the fixture by turning the water valves for both hot and cold water located under the sink. Turn on the faucet to drain any remaining water in the lines. Use an adjustable wrench to disconnect the water supply lines. Now spray the faucet tailpiece, mounting nuts and coupling nuts with WD-40® spray lubricant to loosen them up if they are hard to turn. Detach the coupling nuts first with a basin wrench or channel-type pliers. Next, unscrew the tailpiece mounting nuts and remove the faucet base from the sink. Clean the now exposed surface underneath with mild detergent, a rag, brush or sponge, depending on how dirty it is. If stubborn plumber's putty remains, scrape it away with a putty knife or gently with a razor blade.
Place a small bucket or pot in the under-sink cabinet to catch any water that might spill while you're disconnecting water lines.