"Zen is very simple. Dishwashing time, just wash dishes; sitting time, just sit; driving time, just drive; talking time, just talk; walking time, just walk. That's all. Not special. That is absolutes thinking. When you're doing something, just do it. No opposites. But don't hold. Thinking is OK. Checking is OK. Only holding is a problem. Don't hold. If your mind is not holding anything, it is clear like space. This space is never broken."
- Seung Sahn
It is hard to say where the mastery truly starts, but one thing is certain, you will notice it when it happens. It grows naturally from advanced practice. It can and should be supported by the practice of noticing (coming soon™) as the advanced practice and noticing practice greatly reinforce each other.
It may be obvious by now that any and all objects of perception or mental formations could and should be taken as an object for mindfulness. The whole practice guides us there. As you train yourself more and more with many different objects, the time you spent in mindfulness will grow and your skill will improve.
At a certain point, there will be a breakthrough when mindfulness stops being a practice and becomes a state of being. Instead of being mindful of an object, you will become just mindful. Anything that you perceive and all that you think will naturally be your focus. Your mind will just flow.
You will enter the state of mindfulness, the goal of this practice.
The rest of the practice is just to enable ourselves to stay in that state longer and longer. It can and will have a profound effect on how you perceive the world and your role within it. You will see.
This is of course not the be-all and end-all of meditation - there is no real endpoint for the path as new vistas will open up before you. But now the path will be so much easier.
That being said - once you reach this point, I would dare to say you are at least the stream-enterer. But more about this and other maps of the path later.