The sourdough starter is equal parts flour and water by weight - later on you might learn there are other ratios you can use but lets keep it simple to start with and this is probably the most common method. In the spreadsheet in my original post you will see water of 150g and flour of 150g - that’s from the starter. The additional water and additional bakers flour is not starter.
You start off measuring a small amount of flour on some digital scales, adding some water of the same weight, stirring it until it’s mixed and leaving it. The next day you add the same amount again of both flour and water (ie double it). You keep doing that (doubling it) for a few days or up to a week until you start getting some natural fermentation from the yeast and bacteria in the air. It will start to bubble and you will notice that the starter rises and bubbles after you feed it. Once you have an active starter it can last for years as long as you keep it fed. You can feed it as little as once a week by feeding it, letting it activate a little and putting in straight in the fridge. If you leave it out of the fridge it needs to be fed every 8-12 hours. For a casual baker the fridge method makes more sense. You can make all sorts of things with excess starter like pancakes if you don’t want to bake bread that week.
I would recommend you do a search on youtube and you can watch someone make one. It’s actually as simple as I just described but it helps to watch I guess. Somewhere on this site there would be other threads on the topic too if you do a search. I didn’t use this site to learn about it though. I just did a lot of searches on the internet and watched a few videos. Hopefully this helps a bit - but the videos on youtube would be a lot better.
There’s quite a bit of information to absorb when you first start making sourdough bread but like anything else it’s not so bad once you know how - it’s actually pretty simple - but there’s quite a bit to know behind that “simple” if that makes sense, So I don’t get discouraged by it all and enjoy it because it’s actually fascinating.
Here’s one description of the process:
A few videos from youtube on it (There are many more too):