How it starts ... there are no rules, there is no (fixed) plan. It depends on my mood, it depends on the weather, it depends on the subject, it depends on the chosen medium and colors. I'm not baking a cake, I don't fully know what will be the end result. I want to be surprised.
There are so many variables and at the same time, there are also similarities. There are heuristics and design choices I make when I start a painting.
The imagery I choose always has the potential to be seriously simplified. A simplification of the scene into 3 to 5 tones that represent the difference between the dark parts and the light parts. There has to be contrast. There has to be the possibility to have a good balance between planes and lines. And I assume there are many more similarities that I'm not seeing yet.