Must be at least 18 years old. 2. Completing and signing Form I-864 makes you the sponsor.Affidavits usually need to be notarized or they are not affidavits, but in you case a statement might be enough. I certify that I filed with the court and am serving a copy of this Witness Affidavit on the following people. The main purpose of an affidavit is to provide a written, sworn statement of fact that can be used as evidence in a legal proceeding. A sworn statement, also known as an affidavit or sworn declaration, is a document that presents facts relevant to a legal proceeding. How to fill out an affidavit form? To start the declaration, the person should state their complete name and address, as well as their country of citizenship. How to write a sworn statement. Required to complete the following form.