See your style guide (APA, MLA, etc.) for the approved format for “Unpublished materials,” but ensure you have the following information: Description and date of item; box/folder numbers; collection title; series or call number; repository name; repository city/state/country
For Example: Hurston to Herskovits, Nov 12 1934; Box 12, folder 5; Melville J Herskovits Papers (Series 35/6); Northwestern University Archives, Evanston IL