Easy Dave. Couldnt be simpler mate- either just keep turning into the mountain until you end up sideways on and there you go, or just push the insides of your skiis out whilst pointing the front of them together (a snow plough) - (basically making an arrow with your skiis). this can take longer to stop, but can be effective whilst turning aswell.
The first thing they teach you in ski school is how to stop - fall over... then they'll teach you how to go straight and then how to snow plough - once you can snow plough, you've got your second method of stopping as it slows you down. Toes together tails apart