How do I replace the washer and o-ring in my bath tub faucet?

My home repair book says this is what need's to be done. It is leaking. However, the book says to change it during a shower? I'd prefer to turn the water off. Any other advice is appreciated. Thanks!How do I replace the washer and o-ring in my bath tub faucet?
Here's some link that should help. I am guessing it has three handles. The last link covers other kind of faucets. Hope it helps.How do I replace the washer and o-ring in my bath tub faucet?
You will have to turn the water supply off, depending on what kind of faucet you have, you will have to remove the faucet handle and remove the ';guts'; it should be brass, you will need a wrench and a screwdriver to do this, after removing the guts you should see the o ring, and perhaps a flat washer as well, replace both with correct size and put back together turn the water on and test for leaks.