3 - To generate DISCARD. Nothing is wasted from feeding the starter, I keep the leftovers for crackers, pizza dough, etc. Especially crackers. (Another recipe I'll need to share.)
It's, of course, all AP Flour. I want as close to authentic San Francisco Sourdough as I can get, and, my friends, that means ONLY white flour. I know lots of you love your multi-flour, 12.3% wheat, handful-of-spelt recipes, but the SF sourdough I grew up on and long for is as white as white can be.
Discard all but a quarter cup or 50-75g (does NOT have to be exact). Keep that discard in a separate container. Add 100g of water that's been boiled & cooled or filtered. The water where I live is great, so I'll even use straight from the tap occasionally. Stir to disperse starter in water. Add just over 100g of AP (103-108g is my usual target), and stir just until everything is combined. Slightly more flour than water will increase the sourness over time.
Depending on your kitchen temp, the starter should be at full throttle in 6-8 hours. Mine always doubles in size and occasionally triples. You can speed it up by leaving it in your oven with the light on.
If you're feeding after baking with it (e.g., replenishing) follow the same procedure, and place back in the fridge as soon as you've fed it. There is a point after about three days in the cooler where it will reach the same point as eight hours on the counter. If you've seen it double to triple in volume and feel like baking, go for it. Yes, straight from the fridge.
I've seen lots of bakers recommend two or three consecutive feedings before using the starter for baking. I call shenanigans! Even after a loooong rest (six weeks once) I still got a great rise out of a single feeding. Used it to bake, and everything was fine.
[Pictures: Homepage - what my starter jar usually looks like. Above - what the starter jar looks like after its twice annual cleaning. Bring on the FUNK!]