Generally - expose the screw that holds the tap head on ( the bit that you turn). Undo the screw and pull the tap head off. This will reveal a large hexagon which forms the tap insert - use a good fitting spanner and undo the insert. Look for the rubber washer on the insert bottom. It may be held on by a brass nut or may simply push on. Replace the washer with one of the same size. Screw the insert back in, but turn the tap on a bit so it doesn't crush the washer when you tighten up. Replace the tap head, do the other one and test. Enjoy!
John
October 2009