Choose a topic that centers upon a person, place or event. All of them invite description.
Open with a significant anecdote, rather than a statement of purpose or thesis. In a descriptive essay, you can leave the thesis implied. An implied thesis is not flatly stated, but everything in the essay leads the reader to the see the idea or understand the sentiment at which you aim.
Develop the body of the essay with passages that include action, dialogue and imagery. You need not include everything possible, but rather discuss highlights. If you're describing a person, use anecdotes, events, some physical description and perhaps a favorite quote to bring the person to life. Mention character flaws as well as admirable traits. If you want to describe a place, include the people and objects in it, as well as colors and smells. To describe an event, also include physical details of its location, and mention the people, objects, sounds and smells. Take particular care to describe the action of an event.
Follow the old writing adage: show, don't tell. To do so, use lots of sensory impressions and describe action with strong verbs. "She became sad," tells the reader how a person felt. Compare this to, "her face shattered like glass, and she wept in the full sunlight," which shows the reader what sadness looks like.
Close the essay with a final image and some reflective remarks. Ideally, the final image will work symbolically. The remarks need not explain the essay, though they might. In either case, your final remarks and description should evoke the essay's significance, leaving the reader with a lasting impression.