How should the college essay tie into the rest of the application?
The essay can tie into the rest of the application by explaining some of the "data" present in the rest of the application. For example, why you changed schools, or the reason your grades dropped. However, only do so if the reason is compelling. Also, in some ways you can go into more detail about your extracurricular activities or other accomplishments. Remember that the essay also provides you with a chance to include information not present in the rest of the application, so don't just repeat the same information over again.