Turn off hot and cold water supply at the valves under the sink, and open the faucet to relieve water pressure. Cover the drain with a piece of rag. Pry off or unscrew the plastic cap on top of the faucet handle, using a thin screwdriver or knife, and remove the top-mounted handle screw with a slotted screwdriver.
Lift off the faucet handle. This will expose the packing nut. The packing nut holds the faucet stem in place. Remove the packing nut with large pair of slip-joint pliers or adjustable wrench. Remove the valve stem; this has reverse threads, so unscrew it by turning it clockwise.
Remove the washer lat the end of the stem. The washer is held in place by a small screw; unscrew it with a screwdriver. Twist the screwdriver counterclockwise to remove the screw.
Fasten the new washer into the stem. The new washer must be identical to the old washer. Insert the stem into the faucet, and turn it counterclockwise to reinstall it. Put the packing nut back on, using a wrench.
Reinstall the handle and replace the plastic cap on top of the handle. Turn the water supply back on and check for leaks.