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Shut both valves on the radiator, spread old towels underneath and place vessel to catch water - crack open union nut to rad and open it’s bleed valve to allow water to come out.
If you have a wet vac use this instead to catch all the water as it comes out - no mess.
If you are brave, get large bucket ( gorilla tub) undo both valve unions and lift off rad and tip into bucket - don’t do this one if white carpets! Also check there is not an earth bonding wire before you try this method.
Once rad is off, drain system through the valves.