Your .TEX source file has the same minimum commands as the other document styles, i.e., you must have the following commands as a minimum:
\documentstyle{letter}
\begin{document}
... letters ...
\end{document}
Each letter is a letter environment, whose argument is the name and
address of the recipient. For example, you might have
\begin{letter}{Mr. John Doe \\ 2345 Jones St.
\\ Oakland, CA 91123}
...
\end{letter}
The letter itself begins with the
\opening command. The text of the
letter follows. It is typed as ordinary LaTeX input. Commands that
make no sense in a letter, like \chapter, don't work. The letter
closes with a \closing command.
After the closing, you can have additional material. The \cc command produces the usual "cc: ...". There's also a similar \encl command for a list of enclosures.
The following commands are declarations which take a single argument.
\address{Return address}
The return address, as it should appear on the letter and the
envelope. Separate lines of the address should be separated by \\
commands. If you do not make an \address declaration, then the
letter will be formatted for copying onto your organization's
standard letterhead. If you give an \address declaration, then the
letter will be formatted as a personal letter.
\signature{Your name}
Your name, as it should appear at the end of the letter underneath
the space for your signature. Items that should go on separate lines
should be separated by \\ commands.
\location{address}
This modifies your organization's standard address. This only
appears if the firstpage pagestyle is selected.
\telephone{number}
This is your telephone number. This only appears if the firstpage
pagestyle is selected.
\opening{text}
The letter begins with the \opening command. The mandatory argument,
text, is what ever text you wish to start your letter, i.e.,
\opening{Dear John,}
\closing {text}
The letter closes with a \closing command, i.e.,
\closing{Best Regards,}