Measure out 25 grams of the starter (up to 50 grams), and add double the grams of lukewarm water, stirring with your fingers to mix. Discard unused portion.
Add the same amount of bread or whole wheat flour as you did water and stir to mix.
Place in a large jar and set in a warm spot for 12-24 hours, before placing the lid on and putting in fridge.
Do weekly. (Drain off any liquid that may be on top before measuring out your next 25g.)