How to read a battery hydrometer?

Could you please explain in detail how one can accurately read a battery hydrometer? What are the key steps to follow, and what factors should one consider when interpreting the readings? Additionally, are there any common mistakes to avoid when using a battery hydrometer? Understanding the process of reading a battery hydrometer is crucial for maintaining the health of a vehicle's battery, so I'd appreciate a comprehensive and easy-to-follow explanation.