If you drain out anything that looks as bad as mine then you get as much out as you can with a hosepipe in the radiator neck then fill it with water and caustic flushing fluid. You run the bike up to temperature which pumps the flushing agent around and then drain it all out. Fill with clean water, run bike for a few minutes, drain and fill with coolant. It's explained very clearly in the Haynes manual and probably in the service manual available on here too.

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