We have yet to submit our application, but this is what I asked friends/family to include:
-current date
-your full name
-how we know each other/are related, how long you've known me
-month/year you met (my partner) and the circumstances of how you met
-acknowledge that you have known (my partner) and I to be in a committed relationship since (date), and living together continuously since (date)
-any other information about us..this will help these letters to look more personalized. things such as: if we've been on a holiday together, celebrated a special occasion together, anything else you can think of
-please sign the letter & include a copy of ID (drivers license) to show that the signatures match/prove your identity
-please provide a contact # or email for yourself
Obviously I asked everyone to format it as they pleased, and told them not to worry too much about length etc. I don't think it's necessary to go into too much detail, it's more to provide support your relationship is genuine and is recognized by friends/family. Everyone just addressed "To Whom It May Concern". Hope this helps.