Oh, it's an easy one ...
6oz plain flour
pinch of salt
4 oz butter
2 oz caster sugar
Grease and base-line an 8" round tin, or a baking sheet
Sift the flour into a bowl with the salt, rub in the butter until the mixture resembles breadcrumbs, stir in the sugar and then work the dough together with a knife until it forms a ball of dough
Press the dough into the tin, or roll out into an 8" circle on the baking sheet then crimp the edges, prick all over with a fork
Bung in the oven 180C/GM4 for about 30 - 35 minutes or until pale golden. Sprinkle with extra caster sugar while it's still warm, leave for a few minutes then mark the surface of the shortbread into wedge shapes. Allow to cool completely before removing from the tin.