If it is active and full of bubbles, discard half the starter daily (or give to friends to maintain as follows) and mix in the same quantities of water, then strong white flour as above. Store covered ...
Mix together the flour, sourdough starter and 250ml/9fl oz water in a bowl. Add the sugar and salt. Turn out on to a surface and knead for 10 minutes or until the 'windowpane effect' is achieved ...