changing washer on mixer tap in kitchen

[phildesign]

How do I change a washer on a mixer tap in the Kitchen.

[Whickerman]

http://www.diyfixit.co.uk/nflash/plumbing/Wash er/washer2.htm

About half way down the page if the leak is at the spout base.

This one if the leak is from the tap itself - you'll know the difference as it's dripping from the water outlet:
http://www.crazysquirrel.com/diy/repair-leakin g-tap.jspx

The most important tips I can give you - under no circumstances throw out the old washer until you've brought it with you to the plumbing supply shop to get a replacement - you'd be amazed how many sizes there are, and they all look alike!
Also, put the plug in the sink hole - it'll stop you losing screws etc down the drain.