You can't really prepare for the written portion of the interview... the best way to approach it is to practice creating outlines for an prompt in a short amount of time. Organizing your thoughts in a eloquent and coherent manner is the key to writing an successful essay. I would say read the news articles and columns to learn learn how to write effectively. Seeing how others write will subconsciously help you become a better writer. Additionally, I would just keep up with the news. Hope this helps.
p.s. Remember to use correct grammar