Well, turn off the boiler and the water supply to the central heating header tank up in the loft (usually). There is usually a drain **** at a low point of the central heating pipe system which you attach a hosepipe to then undo the nut. Allow the water to drain outside. After a while (once the header tank has emptied and the top radiators are emptying) undo the bleeder valve in the upstairs radiator to allow them to drain easier. Repeat for the downstairs rads. Remember to do them up when you refill the system.
Ha ha, I see what the member meant in another post,about rude words being edited out! That is silly, but I suppose its better than being offended by deliberate vulgarity.