If they don't accept the Common Application, you may have to fill out a different application for each school on your list. Note any deadlines, and give yourself plenty of time to meet them. Answer ALL questions. Don't leave anything blank. Type or print your answers.
Presentation is important.
Make a copy, in case the original gets misplaced. Include a check for the processing fee, if required. Practice writing the essay before putting the final version on the application. Fine tune your essay. Have someone else review your draft essay.