Warm up. If your starter has been stored in the refrigerator, take it out and let it warm to room temperature for several hours (or overnight) before proceeding.
Dump off the hooch.
Use a clean container. We transfer some of our old starter into a second clean flip-top container before feeding it. If you don’t have a spare container, then temporarily transfer your starter into a clean bowl while you wash the other.
Discard a portion. You only need ½ cup of starter for the next step. So, you can either discard some or put the extra into a different storage container to keep if you wish.
Feed. In a clean container, combine ½ cup sourdough starter, ½ cup lukewarm filtered water, and a scant (light) cup of flour. If you prefer to feed your starter by weight, use equal weights of starter, flour and water (e.g. 100 grams of each). Stir well. Note the starting level on the container so you can track its growth.
Place the starter in a warm location. Sourdough starter is most happy and active around 70-75°F.
WAIT. Be patient. A well-maintained, regularly-fed starter will start to rise within just a couple hours of feeding it. However, an old neglected sourdough starter can take 24 hours or longer to show signs of life. Feeding it too frequently too soon (before it has a chance to wake up and grow) can actually weaken it. So, wait at least 24 hours to feed it again. (That is, unless it rises and falls all the way back down to the starting point sooner – then go ahead and feed it again.)
Discard and feed again. Next, stir the starter and remove all but about half a cup. Repeat Step 5: add ½ cup lukewarm water and a scant cup of flour, mix well, and put the starter back in a warm place. You don’t need to necessarily wait 24 hours to feed it again this time. The starter will tell you when it’s ready – once it rises and falls again.
Repeat a third feeding if needed, especially before baking bread. Once the old starter has been thoroughly reactivated, it will reach peak activity (rise to at least double in size) faster and more vigorously than when it first came out of the fridge.
Once your starter reaches peak activity, it's ready to bake with! Enjoy.