1. In your group, go to the academic databases and work together to find potential sources. Read abstracts, skim articles, discuss. If you aren't finding good academic journal articles, go looking in other very credible places - research findings published on university websites, government data, etc. When you think you've got something, call me over.
2. Read through your sources to find the BEST evidence to include in your letter. BEST evidence will be BRIEF, recent, convincing.
3. Continue drafting the three sections of your letter. Let me know if you have questions!