Find the two oval-shaped water supply shutoff valves under the sink, then turn each valve in a clockwise direction to stop the supply of water.
Pry the index cap from of the handle of the leaky faucet with a flat-head screwdriver to reveal the handle's retaining screw. Be sure not to loosen the single washer just under the outer lip of the cap.
Remove the handle's retaining screw in the center of the handle with a flat-head screwdriver, then pull the handle from the top of the faucet assembly to reveal the handle adapter.
Slide the handle adapter from the top of the faucet valve. The handle adapter is either cylindrical or cone-shaped, depending on the model.
Remove the leaky faucet valve from the body of the assembly. Twist the base of the valve in a counterclockwise direction with a wrench until the valve can be withdrawn from the assembly. Pry the single rubber O-ring from the cartridge with a flat-head screwdriver, then coat a replacement O-ring with plumber's grease. Slide the new O-ring onto the faucet, then insert the faucet into the assembly. Tighten the faucet with a wrench.
Slide the handle adapter onto the top of the faucet, followed by the handle. Insert the handle's screw through the center of the handle, then tighten the screw with a flat-head screwdriver.
Place the handle's index cap over the screw in the center of the handle, then press downward on the cap until it slides into place in the handle. Turn the water supply back on by turning each water supply valve in a counterclockwise direction. The repair is completed.