http://www.youtube.com/watch?v=P-5UVI3nmrY
The steps are very simple: All you need to do is unscrew the electricity supply, making sure you screw it back in once you’re done, to avoid obvious evidence you have tampered with it. Only touch insulated items; avoid the wires. Your hands should be dry while doing this. You need to find a fuse of between 35-40A and the ceramic base that comes with it –both easily and cheaply supplied through electricity stores. Using a power-testing screwdriver (they indicate whether the power supply is live) you can see that the power is still live, which it would have to (otherwise paying customers would lose access to their connection). You insert the fuse into its base (it can only go in one way), then screw both into the meter. Close the lid, screw it closed, leave the ‘disconnected’ sign on… you’re done.
Sources: Sinialo squat, Occupied London