What works for me is to say the things out loud and then write down what I say. Other than that, I don’t worry about it for the first draft. After that, though, it is more art than science. Natural speech is filled with “um” and “ah” and half-completed thoughts. Generally, writers filter all of that actual talking non-sense to make the book easier to read.
My advice is not to worry too much about it while you get the story told.