On Leaven Day (the day before you plan to make your dough), feed your starter first thing in the morning and leave it on the counter. In the evening, you will make your leaven.
In a larger-than-normal mason jar, pour in water. Add starter, close lid tightly and shake to incorporate.
Open the jar and add both flours. Using a bread knife, stir it all together until it’s sticky and there is no dry flour remaining. (Look through the bottom of the jar to make sure there's none hiding!)
Leave on the counter overnight – it should take 8 to 12 hours to ferment appropriately. It will have doubled or tripled in size, and will be light and bubbly with a sweetly sour smell.
To test readiness, take a glass of warm water and drop a small amount of your leaven in it. If it floats, it’s ready for baking. If it sinks, it needs more time to ferment. Recheck every 30 minutes or so.
When it passes the float test, you may begin mixing your sourdough!