Identify the audience for your description. Your description should be tailored toward the people who will be reading it. The audience will determine the vocabulary used, depth of description and the reading level it is written to. For example, you would describe water differently to a group of chemists than you would to a group of elementary school children.
Define the purpose of your description. To achieve an effective description, it is necessary to understand why you are writing. The purpose of the description will govern which details are relevant to the description.
List any relevant facts. These may include physical features, performance features, statistical data, etc. The audience and the purpose of your description will help determine which details are relevant. Ask yourself what the audience would want to know in order to accomplish your purpose. For instance, if you are trying to sell a car to a collector, you will want to include a physical description of the car, details about its engine, perhaps its history, and its price.
Describe each fact clearly. Use your senses -- sight, hearing, smell, touch and even taste -- to illustrate the details relevant to your description. Include details that will help your audience understand the facts, like measurement, perspective or statistics. In creative writing, it is useful to "show" the audience what you are trying to describe by placing the object (person, place, or thing) in a scenario that illustrates its characteristics.