Author, A. A. (Year, Month Day). Title. [Description of material]. Name of collection (Call number, box/file/item number, etc). Name of repository, location.
Example:
Tomlinson, J. T. (1942, April 17). [Memorandum to the deputy minister, J. H. McKechnie]. Saskatchewan Department of Education fonds (Ed. Addendum, file 49, Métis schools). Saskatchewan Archives Board, Regina, Canada.