Use a kitchen scale to transfer 4 oz (113g) unfed 100% hydration sourdough starter into a bowl.
Add 4 oz (113g) filtered or purified water to the starter and mix to combine. The starter will not dissolve entirely, it’s okay if some lumps remain.
Add 4 oz (113g) flour to the starter/water mixture and use a bowl scraper or spatula to combine. Try to break up any large clumps of flour, but again, it’s okay if some smaller lumps remain. You shouldn’t see any obvious dry spots.
Cover the bowl with saran wrap or a clean dish towel and let sit at room temp until bubbles form. Depending on how cold your water and starter were at the start, it may take anywhere from 1 to 4 hours for this to happen.
Meanwhile, clean out the container you usually keep your starter in. Make sure it is dry with no remaining soap residue. Use that discard starter to make scallion pancakes or something. Have fun with it.NOTE: If you’re keeping your unfed starter as a backup, wait to do this until you’re sure the fed starter is bubbly.
Transfer the newly fed, happily bubbling starter to the clean crock. If not using immediately, refrigerate. If you do plan to use it immediately, leave it out at room temp 6-8 hours (or even overnight) until very, very bubbly.