Using a flushing agent such as sentinel X400 (now X800 for power flushing), add the agent to the system and turn your central heating pump to maximum. Run the system for minimum 24 hours to de-sludge the radiators. Then drain off and flush until the water runs clean. Finally, add Sentinel X100 inhibitor to the system to prevent further sludging.
Sorry but the easiest way is to get a central heating engineer or plumber who specailises in C. heating to do it for you. They're the experts and its worth it..