Check it out here but the gist of it is:
Step 1
Turn the mains water supply off.
Step 2
Check that you have really, really done step 1 or you will be in for a wet surprise
Step 3
Make sure you have the correct tools for the job: an adjustable wrench, washers to fit a variety of taps (both inside and outside - they are different) some sealing tape, a tap reseating tool and a black cat for professional advice
Step 4
Undo the tap using the wrench. If you wrap the assembly with an inner tube you can protect any nice, inside taps from scratching.
Step 5
Seek advice on whether or not the washer needs replacing from a more learned species than I.
In this case, the learned one suggested that the split at 5 o' clock prolly needs to be fixed.
Step 6
Replace with a new washer after appropriate approval.
Step 7
Use the reseating tool to ensure a neat surface within the tap assembly.
Step 8
Put the tap back together again
Step 9
Turn the mains water back on.
Step 10
Admire the end result for a total of 5 minutes and the cost of 60c for the washer.
No cats were harmed in the creation of this tutorial - came bloody close, though
There you have it, Doc's 10 Step plan
Love, peace and mungbeans
Doc
Edited by Doc because you use a wrench, not a wench, to undo the tap assembly.
huh? Doc doesn't know how to change a washer??? 
(thanks)
Goodness knows who came up with that clever idea... 
but id find it really hard to explain, cause im crap like that. i just do things haha