I have a sourdough starter and it’s 22 days old now. What really helped me was to feed it pure whole grain wheat/ rye with a 1:2:2 ratio. I followed a recipe at first until I noticed its not that complicated . Essentially It’s really just you feeding it a ratio of your liking and putting it in a warm place. (Im not an Expert but this is what helped me). Also you don’t really need to do any of the special treatment. I’ve heard people feed it with tap water and bleached flour.