No sourdough starter? Use 1/4 tsp of dried yeast instead! If you've got instant yeast (which is called 'fast action dried yeast' in the UK) you can just mix it straight into the starting batter. If you've got dried yeast which needs hydrating first (called 'active dried yeast' in the US) - mix it into the 120 g of water and let it sit for 5 minutes before mixing it into the rest of the 'starting batter' ingredients. Check your packet of yeast for instructions if you're not sure which type of yeast you have.