An affidavit is a written statement made under penalty of perjury or signed under oath before a notary public. If your state requires that a particular document be notarized, the notary signs the document and fixes her seal to it in order to notarize it. She must watch you sign the document.
TL;DR (Too Long; Didn't Read)
To have an affidavit notarized, contact a notary public and arrange to sign the affidavit in front of her. After she observes you signing the affidavit, she'll notarize the affidavit and affix her seal to it as required by your state's laws for notaries.
What Is a Notarized Affidavit?
An affidavit is a written statement you make under oath. The contents of an affidavit are limited in the same way that testimony is limited. That is, you can state facts about which you have personal knowledge but not information you've merely heard from another person. For example, you can say in an affidavit that you saw Jim give Jane an envelope, but you cannot say that Lucas told you that he saw Jim give Jane an envelope.
Similarly, you should stick to firsthand knowledge in your affidavit declarations. Leave out opinions unless you are an expert who has been asked to give expert opinion testimony. And do not include speculation, like "I saw Jim give Jane an envelope that probably had cash in it."
Affidavits are often used when a witness has personal knowledge relevant to an ongoing case, but that person cannot come to court to testify. Witnesses in court swear to tell the truth before they testify. In an affidavit, you state in the document that you declare under penalty of perjury that it is the truth. A notarized affidavit is one in which you swear the content is true before a notary public.
What is a Notary Public?
A notary public is a person who has been appointed by the state to act as an impartial witness for purposes of document signatures. The purpose of a notarized signature requirement is to protect against fraud and authenticate a sworn statement.
How to Notarize an Affidavit
You can find notaries public in many different types of businesses. Legal offices usually have notaries, as do real estate offices, accountants, title offices and banks. Some government agencies also have notaries. Once you have found a notary public, take the affidavit along with photo identification to the notary's office. The notary may request that you put your name, address, the title of the document and your signature in his notary book.
When the notary tells you to proceed, sign and date the affidavit while he watches. When you are done, he will stamp the document and sign it as well. Generally the notary signs the bottom of the document under these or similar words: "Subscribed and sworn before me this __ day of _ , _." It's important that you not sign the document until the notary is available; he must observe your signature or he won't be able to notarize it.