This is a bit different one, since maintaining a sourdough starter is an on-going process rather than a one-and-done recipe. But I'll shove it into my recipe format since I don't have a better idea. I'll leave lots of notes on this one.
Ingredients
? g previous starter
20 g water
20 g flour
Steps
Collect a small amount of starter from your previous batch. This doesn't need to be much, it could be as little as half a gram. I never measure, I just scrap up a bit from the bottom of the jar, it's likely less than 5 grams.
Dissolve your bit of starter into the water. Stir in the flour and mix well.
Let the starter sit for approximately 12 hours, it should more than double in size.
Discard most of the starter, and start again.
Notes
When I'm not going to be baking sourdough in the next couple of days I'll store the starter in the fridge after letting it ferment for several hours. I'm trying to catch as much active yeast when I put it in the fridge. It will keep in the fridge for a long time, but the longer you keep it in the fridge the longer it'll take to bring it back to life. I'll commonly keep it in the fridge for most of the week, but have kept it in the fridge for the better part of a month without problem and I've heard stories of people keeping it in the fridge for over a year and still being able to revive it.
When in the fridge I keep it covered so that it doesn't dry out.
When reviving starter from the fridge I'll get it out 1-2 days before I plan on baking and try and do 2 rounds of feeding and discard before using it to make starter for a recipe. I'll take it out of the fridge and let it come to room temperature for a few hours before discarding and feeding it. If I'm being honest I very commonly only do a single round of feeding when I get it out of the fridge. If I've left it in the fridge for more than a week I'll try and do more feeding cycles to get the yeast back into a healthy state.
Whole wheat flour has more nutrients that white flour, so I try and mix it up every so often and throw in some whole wheat flour when I'm feeding my starter. I don't do it every time, but when I think about it. This might be something good to do when you're preping to bake something with whole wheat flour in it, it'll help acclimate the yeast a bit before they get dumped into the larger dough recipe
I've left my starter in the fridge for several weeks on occasion and have had no issues reviving it. I've also left it out on the counter only covered in a towel while on vacation for a week. I came back and it was mostly dried out and had some questionable bits, but was able to revive it with my usual approach when taking a bit of dried up starter that looked uncontaminated and feeding it for 4 or 5 cycles. Turns out starter is tough to kill!