There is no form or specific letter, but it would look something like this: Dear USCIS Officer, I, (name of US Citizen), and legally and validly married to (beneficiary's name) since (date of marriage). We have not married solely for an immigration purpose nor to avoid the immigration laws.
Copies of the following, showing both spouses' names: Joint mortgage or lease documents (make sure to include a copy of the entire mortgage or lease) Utility or other bills showing both spouses' names. Property deed.
Each affidavit must contain the full name and address of the person making the affidavit; date and place of birth of the person making the affidavit; and complete information and details explaining how the person acquired his or her knowledge of your marriage.
Describe the facts and information that you have firsthand knowledge of. Be as detailed and specific as possible, and make sure that your statements are truthful and accurate. Sign and date the affidavit, and have it notarized by a licensed notary public. The notary will verify your identity and witness your signature.
Each affidavit must contain the full name and address of the person making the affidavit; date and place of birth of the person making the affidavit; and complete information and details explaining how the person acquired their knowledge of your marriage;