Now that your mixture is at room temperature, it's time to introduce the yeast. Gently pour in 1-2 teaspoons of active dry yeast, or 1/4 cup of rehydrated yeast. Make sure not to overdo it, or you'll end up with a batch of sour, slightly explosive bread.
Yeast: The unsung hero of the sourdough world.
Proceed to Step 3: The Long, Dark Wait, where you'll be rewarded with the sweet taste of anticipation and the bitter taste of doubt.
Or, if you're feeling adventurous, try skipping ahead to Step 4: The Unexpected Explosion