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} \signature{Your Name} \address{Return address} \begin{document} ... First letter ... ... Second letter ... ... \end{document}Note that the \address and \signature commands are declarations and follow the usual scope rules. They are generally placed in the preamble, as indicated here, and apply to all letters in the file.
Each letter is a letter environment, whose argument is the name and address of the recipient. For example, a typical letter might have
\begin{letter}{Mr. John Doe \\ 2345 Jones St. \\ Oakland, CA 91123} \opening{Dear John,} ... text of letter ... \closing{Best regards,} \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.
\renewcommand{\today}{Month day, year}This may be put in the preamble if it is to apply to all letters in the file, or just before the \opening command if it is to apply to just that letter.