Replace Washer or O-Ring

First, turn off the water supply valves under the sink. Open the faucet to drain remaining water. Carefully disassemble the faucet handle and stem using appropriate wrenches and screwdrivers. Identify the worn washer or O-ring (often black rubber pieces) causing the leak. Take the old part to a hardware store to find an exact match. Replace the worn part, reassemble the faucet tightly, and turn the water supply back on, checking for leaks.