An affidavit of truth is your personal statement of the facts as you know them, sworn under your own hand, with the lawful weight of an oath behind it. It is the heaviest piece of paper you can produce on your own, without going through their courts.
An affidavit is not a letter. It is a statement of fact, written in the first person, signed in front of a witness, and sworn to be true to the best of your knowledge. The classical position is that an unrebutted affidavit stands as truth in commerce and in lawful matters. If you state your facts, swear them, serve them on the other side, and they do not rebut them point by point under their own oath, those facts are taken to have been admitted by silence.
When to use one
Use an affidavit when you need to put your version of events on the record in a way that cannot easily be brushed aside. Examples: after a dispute over council tax where the council has refused to identify the living claimant, after a stop by a constable where you want to put your account on record before any court date, or as the foundation for a Notice of Default following an unanswered conditional acceptance.
Do not use an affidavit for trivial matters or as a first move. It loses its weight if you fire one off every week. It is a serious document. Treat it that way.
Structure
Heading: Affidavit of Truth. Your name as a living man or woman, in lower case, followed by the words "in propria persona". A numbered list of statements of fact, each one short, each one something you can stand on under oath. A closing paragraph: "I, [name], do solemnly and sincerely affirm that the foregoing is true to the best of my knowledge, information and belief." Signature in blue ink, with your autograph, and the signature of a witness, who can be any adult who is not a party to the matter.
Hold the line
The other side will, in most cases, not respond to your affidavit at all. Their staff are not trained to swear counter-affidavits, and their solicitors will usually advise them to ignore the document and proceed under statute regardless. That ignoring is itself useful. Every unrebutted point becomes part of your record. If they later try to take you into their court, you walk in with a sworn document on the file that has never been answered.
Keep three copies. One sent recorded delivery. One stored at home. One kept with a trusted third party. The point is that the document survives even if your house burns down or your post is intercepted.