You don't - you adjust the inlet valve.
Remove the lid from the cistern and flush several times as you observe the mechanism.
Pull up on the float and see the incoming water stop flowing. Let go of the float and watch the water run into the overflow tube.
Adjust the inlet valve screw inwards to shut off the flow sooner - set the water level to half an inch below the level of the overflow pipe. Be sure to tighten up the little nylon locking nut.
Good luck...
Peccavi, May 2013
