Short answer. You must prove its due execution and authenticity before it is received. Rule 132, Section 20 allows three ways: testimony of anyone who saw the document executed or written, evidence of the genuineness of the maker's signature or handwriting, or other evidence showing its due execution and authenticity.
What the law says
Before any private document offered as authentic is received in evidence, its due execution and authenticity must be proved by any of the following means: (a) By anyone who saw the document executed or written; (b) By evidence of the genuineness of the signature or handwriting of the maker
Rule 132, Section 20 — Proof of private document[s]. Read the full provision →
Private documents do not prove themselves
A private document — one that is not a public or notarial record — carries no presumption of genuineness. Rule 132, Section 20 is explicit: Before any private document offered as authentic is received in evidence, its due execution and authenticity must be proved. Until that showing is made, the unnotarized contract, receipt, letter or promissory note you are relying on is just a piece of paper; the court cannot treat it as the act of the person who supposedly signed it. This is why simply attaching a document to a pleading, or marking it at trial, is not the end of the work.
The three recognised ways to authenticate
The rule gives three routes, and any one suffices. First, anyone who saw the document executed or written may testify — a signatory, an instrumental witness, the person who prepared it. Second, evidence of the genuineness of the signature or handwriting of the maker: someone familiar with the maker's handwriting, or a comparison with admitted or proved specimens. Third, a catch-all — other evidence showing its due execution and authenticity, which leaves room for circumstantial proof where no eyewitness or handwriting evidence is available. Plan which route fits your document before trial, because the witness you need must actually be presented.
Only documents offered as authentic need this
Notice the qualifier: the requirement attaches to a private document offered as authentic — one you ask the court to accept as genuinely made or signed by its maker. The same section adds that any other private document need only be identified as that which it is claimed to be. A document offered for some other purpose — say, merely to show that a paper of that description exists or was received — does not demand the full authentication exercise. Being clear about the purpose of your offer therefore decides how much foundation you must lay.
The ancient document exception
The Rules excuse authentication entirely for so-called ancient documents. Where a private document is more than thirty years old, is produced from the custody in which it would naturally be found if genuine, and is unblemished by any alterations or circumstances of suspicion, no other evidence of authenticity is required. Old family papers, decades-old deeds and long-kept correspondence can qualify — provided all three conditions hold, including the tell-tale one about custody: the document must surface from where a genuine one would naturally be kept. If any condition fails, you are back to proving authenticity by the ordinary three means.