Plenty of sources will tell you that when you’re baking your first sourdough loaf, it’s best to begin with a nice, stable starter you’ve acquired from someone else. It’s reasonable advice. However, this is an adventure we’re beginning on, and if someone tells you to hitchhike the first mile, you don’t have to listen to them.
Every source disagrees with every other one on exactly how to make a starter. That’s because the mechanism is mysterious: do the yeast and lactobacilli come from the flour, the air or those halved grapes you added? But take comfort: it’s also a piece of piss.
This is the method I used. It’s a thorough but not too prissy explanation. I haven’t much to add, except that you need to be patient. How fast your starter stabilises will depend on your local conditions. Mine took a week to get stable in British winter. Also, you eventually need to be feeding the starter in the ratio 1:1:1 of weight of starter:flour:water every 12-24 hours. It seems excessive, but this is bacteria we’re talking about: eating things is its super-power.
At the top: a picture of my starter in full bubbling flow.