A hydrometer measures the weight, or gravity, of a liquid in relation to the weight of water. This is useful in the brewing of beer because the hydrometer will float higher in a liquid with sugar dissolved in it and lower in a liquid like alcohol. Since yeasts converts sugar into carbon dioxide and alcohol during the brewing process, you can calculate how much alcohol resulted by knowing the amount of sugar you started and ended with.
A good first step for using the hydrometer is to sanitize everything involved and then to place the test cylinder on a flat surface. Then take a sample of beer or wine. If using wine, make sure that it does not contain any particles because they will affect the reading.
Next, fill a jar with enough of the beer or wine to make the hydrometer begin to float. Slowly lower the hydrometer into the test jar, making sure to spin the hydrometer as you release it. This prevents bubbles from sticking to the bottom of the hydrometer, which can affect the reading.
You should also make sure that the hydrometer is floating freely and not touching the sides. Take the reading from the bottom of the curve of the liquid, also called the meniscus. It is important to note that hydrometers are generally calibrated to give readings at 59-60 degrees, and any difference can lead to some error in the readings.