Each institution will vary in how they'd like you to cite archival sources! The important thing is to include enough information that if a stranger read your paper and wanted to look at the same source that you did, they would be able to find the resource. At Wardman Library, our preferred citation format is:
Title of the file or item, author/creator's first name middle name last name [if known, otherwise omit]. Day month year of production/publication [if known, otherwise write "Date unknown"]. Box/Cabinet number, Drawer and/or Folder number, Collection Name. Wardman Library Special Collections and Archives, Whittier College, Whittier, CA.
For other useful tips, check out this libguide by Cal State University, Dominguez Hills: https://libguides.csudh.edu/archives-citation/about.