TL,DR: You can't.
From my limited understanding of the brain and experience of other patients. There is no such thing.
The subconscious mind is automatic. It actually controls most of the part of the brain. The "thinking part" is merely the tip of the iceberg. Your brain actually doesn't quite belong to you.
If you are able to even "control" a tiny part of the thinking brain, you would already be a genius. Anything more, you might already be Einstein or Elon Musk. The sad fact remains that, most of us don't use much of our brain. (LOL) We rely on patterns, routines and the "primitive" brain to make most of our decisions.
You could try meditation, to try to "see" what the mind is thinking. And perhaps that could be the only window to try to calm the monkey mind.