Helpful tip: When positioning the mouse, be sure that your elbow is away from your torso; i.e. arms out. This will limit the amount of forearm rotation (pronation) which causes strain and potentially carpal tunnel syndrome. The mouse pad should be placed farther to the side of your body. The same applies to the keyboard; it should be placed to the side, not in front, which reduces pronation movement.
More detailed explanation: when your arms are positioned close to your body, your forearms have to rotate more to hold the mouse and keyboard. But as the arms move away from the body, the palms naturally face downward. This reduces the amount of pronation to hold the mouse and press the keyboard.
EDIT: drew a diagram of this to help visual: https://imgur.com/a/CduKUJD