How do I flush cooling system on 1993 Corsa?

I have recently purchased a 1993 L reg corsa with only 32k miles. It rapidly runs out of coolant and engine thermostat says over 100 when in traffic. I have to top up with 2litres of water every other day. also has just stopped blowing hot air in the car, only cold air blows now. Any ideas whats wrong? also how do i flush the cooling system and wash the radiator?