Removing the body
Dismantle the head of the tap - if the tap has a metal cover then this needs to be removed - be sure to protect the taps surface from any tool, ideally unscrew by hand. A block of wood to support the spout can be quite useful for opposing any forces applied with a spanner. With the cover removed, the nut which contains the tap spindle and washer should be revealed, use a spanner to unscrew and lift out the tap assembly from the base and spout.
Replacing the washer
The washer should be at the bottom of the removed assembly. The washer is attached in one of two ways; it is either held in place with a small nut or pushed on over a centre point. If the washer is held in place with a nut then it can be very difficult to remove in this case oil the nut with a penetrating oil to ease any corrosion. Once the old washer has been removed fit the new washer.
Reassemble the tap in reverse order.