If it's a grand piano, mount the condenser mic on the lid near the player, and above middle C.

If it's an upright, I'd open the front lid and mount the mic in front of the player's head above middle C.
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Alright, theres a few ways to go about this.

1 way for an alright, all around sound is to put the mic about 3 feet back, and 7 feet off the ground, pointing down at the centle of the piano. Have the top lid open, and take off front access panel (if you dont know how I can tell you).

Way 2 is to put a large diaphragm dynamic or smal diaphragm condenser under the keybed, about 1 foot below the action, 1 foot back, pointing up into the hammers. This way is more punchy and in your face.

Way 3 is to open the lid and suspend a large diaphragm condenser about 2 feet above the top of the piano, around the midddle, taking care not to point directly at the hammers.