I had the people who wrote about our relationship write casually and in their own words about what they know about our relationship. Some told a chronological story. Some just told the relationship details. All I asked was that they use their own knowledge and wording. My mother in law wrote a very sweet letter admonishing Immigration to just let us be together.
My best friend wrote about how she was initially sceptical, but over the years has come to see us as the most loving and close couple she knows (despite the LDR for most of our relationship). One of our wedding witnesses wrote about our wedding day and then went into her knowledge of our relationship after our wedding.
Oddly, two of the more creative people I know couldn't figure out how to write it, and we had to send out our application without it. Oddly, we didn't get any of the letters (as well as some other odds and ends) back, but I don't know if it's because they couldn't afford to send it. With our communication details (6 years of phone records), our binder cost us $50 to overnight priority to NYC.