Lawsuits involve applying law to fact; at the heart of any lawsuit, therefore, is a mantra from "Dragnet's" Joe Friday: "Just the facts, ma'am." Litigants have multiple ways to present facts; two common forms are witness testimony and affidavits. An affidavit is a written document, signed under oath or affirmation. Because the affidavit is a sworn document, the affiant (the person who gave the statement) is guilty of perjury if he lies in the document. Affirm an affidavit by signing it under oath.
Draft the affidavit. In the affidavit, include the facts and circumstances pertinent to the case. Facts generally include the name and birth date of the affiant and a description of what the affiant saw or witnessed.
Take the document to court or to a notary public. Ask to be sworn under oath. You must be sworn under oath by a person authorised to do so (such as a county clerk or a notary public).
Sign the document under oath and in the presence of the person authorised to swear you in. Signing the affidavit under oath affirms the affidavit; it is your personal guarantee (under penalty of perjury) that the document contains the truth.
Notarise the document or have the document signed by the person who swore you in.
Affirming an affidavit is relatively uniform across multiple legal systems; the references below describe very similar processes in the US, Canada, and Australia. Review the sample affidavit form to familiarise yourself with what an affidavit looks like.