Find a tub or jar to make it in, must be able to seal it.
Day one - mix together a tablespoon of strong white flour with a tablespoon of water. Mix it together with your finger (washed of course!).
Day two - add two tablespoons of the same flour and two tablespoons of water to the day one mix and stir together.
Day three - add three tablespoons of the same flour and three tablespoons of water to the mix and stir together.
Day four to seven - each day add three tablespoons of the same flour and three tablespoons of water and mix together. After day four you should hopefully start to see some bubbles forming in the mixture.
You now have your starter. Your starter can be kept out at room temperature or in the fridge.
If you keep out at room temperature you must feed it each day with a couple of tablespoons of flour and water.
If you keep in the fridge then you can leave it for two weeks (three weeks at an absolute maximum). Every two weeks remove from the fridge, feed with a couple of tablespoons of flour and water, leave at room temperature for 24 hours, feed again with a couple of tablespoons of flour and water and then pop back in the fridge for 2 weeks.