Pinch off about ¼ cup of sourdough and place in a glass jar or non-reactive container.
With a fork work in 1 cup of flour or more until the dough is very stiff, dry, and crumbly. Heap some more flour on top.
Place a lid on the jar and place the sourdough starter in the refrigerator.
The evening before you want to bake, take the jar out of the refrigerator.
Add some filtered or distilled water to it. With a fork, stir the water into the sourdough starter until you have a thick consistency. Don't worry about any remaining clump.
Let this mixture sit out at room temperature until the next morning.
Use your active sourdough starter according to the recipe.
Don't forget to keep back ¼ cup of starter to repeat the process.