A man is walking down the street in NYC when a very harried looking person runs up to him and asks "How do you get to Carnegie Hall?"
The first man answers "Practice."
Get to the point where you can hit one spot consistently. I usually recommend low and down the middle to start because it should be easiest and makes a good foundational starting point. Then make adjustments to go left/right and up/down. As others have said, up/down is posture. Make down the default, then I usually have pitchers tilt back slightly to raise the ball without changing the rest of their mechanics. For left/right, I like striding *slightly* left or right of where you stride to hit center. The operative word being slightly. You shouldn't be able to see the difference from the batter's box.
The first man answers "Practice."
Get to the point where you can hit one spot consistently. I usually recommend low and down the middle to start because it should be easiest and makes a good foundational starting point. Then make adjustments to go left/right and up/down. As others have said, up/down is posture. Make down the default, then I usually have pitchers tilt back slightly to raise the ball without changing the rest of their mechanics. For left/right, I like striding *slightly* left or right of where you stride to hit center. The operative word being slightly. You shouldn't be able to see the difference from the batter's box.