Discard half of the starter(can be used for pancakes and etc.) Add a tablespoon of apple cider vinegar and a tablespoon of potato flour; and 1\2 cups flour and 1\2 cups water. Let proof until bubbly and the either use or store in the fridge.
Before using the starter, take it out of the fridge and and mix equal, by weight, flour and water in a bowl. It depends on how much you need for the recipe. If the recipe uses a cup of starter, use 120G of water and 120G of flour. Add a tablespoon or two of starter and let it proof for several hours or so-- you will see the mixture rise and get bubbly. It is now ready to use. You can use the old starter for pancakes, crumpets and etc.
The left over starter is saved for the next batch after adding to the recipe. It can be stored in the fridge for up to two weeks.