For a 100% hydration starter (this means the water weighs the same as the flour) you would use equal amounts, by weight, of the ripened starter, flour, and water. You could take one ounce starter and add one ounce of water and one ounce of flour - reserve the rest of the old starter in the fridge as a back-up, just in case. To cut down on wasting flour and having my starter take over the entire kitchen, I usually do a small amount like this until I am planning to use it. Then I will give it frequent feedings and increase the amount according to how much I will need for the recipe.