Include your contact information on the document. This could be on official letterhead or simply just at the top left of the page. Letterhead is helpful if you are an employer or landlord to the subject of the letter. This will give weight to your letter, which can better the individual's chances at having her residency reinstated.
Address the letter to one person or organization specifically. This will ensure the letter gets into the right hands. Addressing to one person's name in particular will also make the letter more personal and therefore more effective. This may take research to be sure the letter reaches the right person. A salutation such as "Dear Mr. Jones" followed by a colon will suffice.
Introduce yourself and why you are writing the letter. For example, "My name is Anna Smith. I am writing to help prove the residence of Sarah Martin. As her current landlord, Sarah has lived in my rental property for 18 months." Be as specific as possible, but concise and pithy. This is an official document, so it should be as straightforward as possible.
Substantiate your claim with proof. For example, give move-in dates that prove that the resident has lived at this address for a certain length of time. Include supplemental paperwork, such as a copy of a lease or pay stubs that prove that the individual has lived in the area for the required amount of time.
Close your letter by thanking the reader for his time. Make yourself available to answer any questions or concerns.