As I expect you to have some playing experience already and, therefore, your own habits and preferences, my rules for hand position are minimal:
- Try to keep the hand, wrist, and forearm in a straight line. This correctly aligns the muscles and relaxes the hand position. Relaxtion will develop only with correct muscles aligment. This applies to both hands.
- Make sure you can see the knuckles in both hands when you play. To do this, you’ll need to have a uniform curve in each finger. Each hand needs to be in a neutral (midrange) position; bending the wrist on either hand tends to flex or extend the muscles, with your left-hand thumb on or near the center of the guitar neck, in line between the index and middle fingers.
Practice in front of a mirror during the first few months of playing the classical guitar so that you can visually check your hand positions and posture. Once these techniques feel comfortable and are internalized, lose the mirror.