Best way, I think is combining a small amount of flour and water to start, maybe 15 grams each. There’s no reason to waste a lot. After that’s had a couple days of room temperature in a jar, you can discard half (use in something else) and add 15 grams of flour and water again. A few iterations of that should get you a usable starter, but it takes a while for it to settle into being stable and predictable, maybe even months, depending on how often you feed it, and what temperature it’s at. Put a rubber band around the jar where the level of the starter is when you feed it, and you can time how long it takes for it to ferment. Ideally, you’d feed it right when it gets to it’s highest level, but no one probably does that unless they are a baker who wants super consistent dough.

The type of flour doesn’t necessarily matter, but I would just use whichever flour you want to bake bread out of. Theoretically, the bacteria and yeast will reach some sort of equilibrium with the nutrients available in that specific flour.

Lots of guides online will have you keeping a fairly large volume of starter, and dumping it right into your dough. That’s the way you’d want to do it if you were a pro baker who had to make bread every day, but for most of us, it’s kinda wasteful. I keep about 60 grams of starter going in a little jar, and I just pull off half and step it up over one day to use the next. I never discard any starter.

Lastly, don’t follow any guides that give you exact times for anything. It’s all so dependent on your starter and the temperature in your kitchen. You need to only look at how it rises and adjust from there. Don’t be too intimidated, though, people figured out sourdough thousands of years ago. The worst thing that happens is that it doesn’t look as pretty as you’d like.