choose a restaurant, library, theater, park, grocery store, or any other place where you can sit and just think about the experience of being there for a little while. Think about what makes the strongest impression on you.
Write up your notes in a paper that is about 3 pages long. You can organize it however you want, though it might be easiest to write a paragraph for each sense (sight, smell, etc) or perhaps just describe the "event" as you experienced it in time and include relevant details as you work through it (e.g. if you choose a restaurant, talk about the experience from first walking in the door to leaving afterwards). Focus on the details that will let a reader understand the experience as strongly as possible. You can write in either present or past tense, but be consistent.
If it helps to think of it this way, the Description Paper assignment is basically a summary of a place--focus on the main points and leave out unnecessary stuff.