First, most importantly, you must know what you are making a picture of, what is important, what you want to show and what you want the viewer to see.
I don't mean in a general sense like 'this is a pretty meadow' or 'here is a nice street scene' or 'there are flowers'.
You should/must know exactly what part of the scene is important because the whole intent of processing is to maximize the impact of the important parts of the scene and, of course, to minimize anything that is either not-important or even actually disruptive to the idea or impression you want to get across.
So you must be able to look very objectively at each picture, look with an intention of discarding pictures if they can't be edited to a good final image.