You will finalize a couple's marriage license after the ceremony through the online Officiant Submission and Affidavit. Must be at least 18 years old. 2.An affidavit of marriage is a legally binding document that proves a legal marriage exists between two people. An affidavit is a written statement made under oath that is typically used in legal proceedings. Learn their purpose, when to use, and how to write. 1. Apply for your marriage license online 2. Send your digital license to your officiant 3. An affidavit is a sworn statement of fact that can be used in a variety of legal proceedings, from bankruptcy cases to family and civil litigation matters.