The phrase "attach a letter" is correct and usable in written English.
You can use it when instructing someone to include a letter as an attachment in an email or document.
Example: "Please remember to attach a letter of recommendation when you submit your application."
Alternatives: "include a letter" or "enclose a letter".
They hoped to attach a letter to the gates outside No 10, as occurred last year.
Attach a letter that explains the problem, and provide copies (not originals) of any supporting documentation, like a canceled check illustrating that you made a payment.
Mr. Berning noted that Maner did not attach a letter with recommendations to its latest audit, covering 2001, "which we understood to be a clean bill of health".
However, Princeton will attach a letter detailing the previous policy and its repeal to the transcripts of sophomores, juniors, and seniors.
Attach a letter of intent that explains what the research/travel award will be used for and how it will support your progress toward completing your master's degree.
If your transcripts or other supporting documents were issued in another name, also include that name on your application and attach a letter of explanation and evidence of the name change.
I also attached a letter from his father to me explaining how important it is to document presidential decision-making.
In papers submitted to United States District Judge Sam Sparks, lawyers for Armstrong, the seven-time Tour de France winner, attached a letter from a lawyer for USA Cycling.
An Iraqi woman who had worked for the Coalition Provisional Authority attached a letter of recommendation written in 2003 by Bernard Kerik, then Iraq's acting Minister of the Interior.
It attached a letter from SPI's executive director, the Rev David Applin, received by the commission in 2001 following previous complaints.
During the war, when an exhausted Martin Tytell was choosing the keys for a Burmese typewriter, he accidentally attached a letter upside down.
