Locating the washer
Close off the water supply to the tap and open the tap to drain excess water.
Put the plug in the waste outlet - not to stop loss of water, but to safeguard against losing any of the tap components when dismantling it.
Remove the top cover of the tap. This may be a metal type with a hexagonal base, in which case you should wrap some cloth around it before undoing with a spanner. The cloth will prevent it being scratched. Hold the tap itself to prevent it turning.
Some taps have a moulded plastic cover held in place with a small screw located in its top (sometimes under a plastic insert, which will need to be gently prised out with a thin bladed screwdriver).
Undoing the tap
Once the cover is removed, undo the top section of the tap by turning the hexagonal nut next to the base with a spanner.
Hold the tap itself (wrapped in a rag) using a pipe wrench to prevent it turning.
Lift the top section out of the tap.
Replacing the washer
On the underside of this will be the washer retained by a small nut, which should be undone, or a button.
Remove the old washer and replace with a new one. Make sure that pieces of the old washer are not left in the seat of the tap before finally reassembling it and switching the water back on.
Tap washer replacement video